Bluetooth: Use lock_sock_nested in bt_accept_enqueue

Fixes this warning that was provoked by a pairing:

[60258.016221] WARNING: possible recursive locking detected
[60258.021558] 4.15.0-RD1812-BSP #1 Tainted: G           O
[60258.027146] --------------------------------------------
[60258.032464] kworker/u5:0/70 is trying to acquire lock:
[60258.037609]  (sk_lock-AF_BLUETOOTH-BTPROTO_L2CAP){+.+.}, at: [<87759073>] bt_accept_enqueue+0x3c/0x74
[60258.046863]
[60258.046863] but task is already holding lock:
[60258.052704]  (sk_lock-AF_BLUETOOTH-BTPROTO_L2CAP){+.+.}, at: [<d22d7106>] l2cap_sock_new_connection_cb+0x1c/0x88
[60258.062905]
[60258.062905] other info that might help us debug this:
[60258.069441]  Possible unsafe locking scenario:
[60258.069441]
[60258.075368]        CPU0
[60258.077821]        ----
[60258.080272]   lock(sk_lock-AF_BLUETOOTH-BTPROTO_L2CAP);
[60258.085510]   lock(sk_lock-AF_BLUETOOTH-BTPROTO_L2CAP);
[60258.090748]
[60258.090748]  *** DEADLOCK ***
[60258.090748]
[60258.096676]  May be due to missing lock nesting notation
[60258.096676]
[60258.103472] 5 locks held by kworker/u5:0/70:
[60258.107747]  #0:  ((wq_completion)%shdev->name#2){+.+.}, at: [<9460d092>] process_one_work+0x130/0x4fc
[60258.117263]  #1:  ((work_completion)(&hdev->rx_work)){+.+.}, at: [<9460d092>] process_one_work+0x130/0x4fc
[60258.126942]  #2:  (&conn->chan_lock){+.+.}, at: [<7877c8c3>] l2cap_connect+0x80/0x4f8
[60258.134806]  #3:  (&chan->lock/2){+.+.}, at: [<2e16c724>] l2cap_connect+0x8c/0x4f8
[60258.142410]  #4:  (sk_lock-AF_BLUETOOTH-BTPROTO_L2CAP){+.+.}, at: [<d22d7106>] l2cap_sock_new_connection_cb+0x1c/0x88
[60258.153043]
[60258.153043] stack backtrace:
[60258.157413] CPU: 1 PID: 70 Comm: kworker/u5:0 Tainted: G           O     4.15.0-RD1812-BSP #1
[60258.165945] Hardware name: Freescale i.MX6 Quad/DualLite (Device Tree)
[60258.172485] Workqueue: hci0 hci_rx_work
[60258.176331] Backtrace:
[60258.178797] [<8010c9fc>] (dump_backtrace) from [<8010ccbc>] (show_stack+0x18/0x1c)
[60258.186379]  r7:80e55fe4 r6:80e55fe4 r5:20050093 r4:00000000
[60258.192058] [<8010cca4>] (show_stack) from [<809864e8>] (dump_stack+0xb0/0xdc)
[60258.199301] [<80986438>] (dump_stack) from [<8016ecc8>] (__lock_acquire+0xffc/0x11d4)
[60258.207144]  r9:5e2bb019 r8:630f974c r7:ba8a5940 r6:ba8a5ed8 r5:815b5220 r4:80fa081c
[60258.214901] [<8016dccc>] (__lock_acquire) from [<8016f620>] (lock_acquire+0x78/0x98)
[60258.222655]  r10:00000040 r9:00000040 r8:808729f0 r7:00000001 r6:00000000 r5:60050013
[60258.230491]  r4:00000000
[60258.233045] [<8016f5a8>] (lock_acquire) from [<806ee974>] (lock_sock_nested+0x64/0x88)
[60258.240970]  r7:00000000 r6:b796e870 r5:00000001 r4:b796e800
[60258.246643] [<806ee910>] (lock_sock_nested) from [<808729f0>] (bt_accept_enqueue+0x3c/0x74)
[60258.255004]  r8:00000001 r7:ba7d3c00 r6:ba7d3ea4 r5:ba7d2000 r4:b796e800
[60258.261717] [<808729b4>] (bt_accept_enqueue) from [<808aa39c>] (l2cap_sock_new_connection_cb+0x68/0x88)
[60258.271117]  r5:b796e800 r4:ba7d2000
[60258.274708] [<808aa334>] (l2cap_sock_new_connection_cb) from [<808a294c>] (l2cap_connect+0x190/0x4f8)
[60258.283933]  r5:00000001 r4:ba6dce00
[60258.287524] [<808a27bc>] (l2cap_connect) from [<808a4a14>] (l2cap_recv_frame+0x744/0x2cf8)
[60258.295800]  r10:ba6dcf24 r9:00000004 r8:b78d8014 r7:00000004 r6:bb05d000 r5:00000004
[60258.303635]  r4:bb05d008
[60258.306183] [<808a42d0>] (l2cap_recv_frame) from [<808a7808>] (l2cap_recv_acldata+0x210/0x214)
[60258.314805]  r10:b78e7800 r9:bb05d960 r8:00000001 r7:bb05d000 r6:0000000c r5:b7957a80
[60258.322641]  r4:ba6dce00
[60258.325188] [<808a75f8>] (l2cap_recv_acldata) from [<8087630c>] (hci_rx_work+0x35c/0x4e8)
[60258.333374]  r6:80e5743c r5:bb05d7c8 r4:b7957a80
[60258.338004] [<80875fb0>] (hci_rx_work) from [<8013dc7c>] (process_one_work+0x1a4/0x4fc)
[60258.346018]  r10:00000001 r9:00000000 r8:baabfef8 r7:ba997500 r6:baaba800 r5:baaa5d00
[60258.353853]  r4:bb05d7c8
[60258.356401] [<8013dad8>] (process_one_work) from [<8013e028>] (worker_thread+0x54/0x5cc)
[60258.364503]  r10:baabe038 r9:baaba834 r8:80e05900 r7:00000088 r6:baaa5d18 r5:baaba800
[60258.372338]  r4:baaa5d00
[60258.374888] [<8013dfd4>] (worker_thread) from [<801448f8>] (kthread+0x134/0x160)
[60258.382295]  r10:ba8310b8 r9:bb07dbfc r8:8013dfd4 r7:baaa5d00 r6:00000000 r5:baaa8ac0
[60258.390130]  r4:ba831080
[60258.392682] [<801447c4>] (kthread) from [<801080b4>] (ret_from_fork+0x14/0x20)
[60258.399915]  r10:00000000 r9:00000000 r8:00000000 r7:00000000 r6:00000000 r5:801447c4
[60258.407751]  r4:baaa8ac0 r3:baabe000

Signed-off-by: Philipp Puschmann <pp@emlix.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>