Bluetooth: Fix possible deadlock in rfcomm_sk_state_change

syzbot reports a possible deadlock in rfcomm_sk_state_change [1].
While rfcomm_sock_connect acquires the sk lock and waits for
the rfcomm lock, rfcomm_sock_release could have the rfcomm
lock and hit a deadlock for acquiring the sk lock.
Here's a simplified flow:

rfcomm_sock_connect:
  lock_sock(sk)
  rfcomm_dlc_open:
    rfcomm_lock()

rfcomm_sock_release:
  rfcomm_sock_shutdown:
    rfcomm_lock()
    __rfcomm_dlc_close:
        rfcomm_k_state_change:
	  lock_sock(sk)

This patch drops the sk lock before calling rfcomm_dlc_open to
avoid the possible deadlock and holds sk's reference count to
prevent use-after-free after rfcomm_dlc_open completes.

Reported-by:  <syzbot+d7ce59...@syzkaller.appspotmail.com>
Fixes: 1804fdf6

 ("Bluetooth: btintel: Combine setting up MSFT extension")
Link: https://syzkaller.appspot.com/bug?extid=d7ce59b06b3eb14fd218 [1]

Signed-off-by: Ying Hsu <yinghsu@chromium.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>