net: thunderx: fix NULL pointer dereference in nicvf_open/nicvf_stop

When a bpf program is uploaded, the driver computes the number of
xdp tx queues resulting in the allocation of additional qsets.
Starting from commit '2ecbe4f4 ("net: thunderx: replace global
nicvf_rx_mode_wq work queue for all VFs to private for each of them")'
the driver runs link state polling for each VF resulting in the
following NULL pointer dereference:

[   56.169256] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000020
[   56.178032] Mem abort info:
[   56.180834]   ESR = 0x96000005
[   56.183877]   Exception class = DABT (current EL), IL = 32 bits
[   56.189792]   SET = 0, FnV = 0
[   56.192834]   EA = 0, S1PTW = 0
[   56.195963] Data abort info:
[   56.198831]   ISV = 0, ISS = 0x00000005
[   56.202662]   CM = 0, WnR = 0
[   56.205619] user pgtable: 64k pages, 48-bit VAs, pgdp = 0000000021f0c7a0
[   56.212315] [0000000000000020] pgd=0000000000000000, pud=0000000000000000
[   56.219094] Internal error: Oops: 96000005 [#1] SMP
[   56.260459] CPU: 39 PID: 2034 Comm: ip Not tainted 5.1.0-rc3+ #3
[   56.266452] Hardware name: GIGABYTE R120-T33/MT30-GS1, BIOS T49 02/02/2018
[   56.273315] pstate: 80000005 (Nzcv daif -PAN -UAO)
[   56.278098] pc : __ll_sc___cmpxchg_case_acq_64+0x4/0x20
[   56.283312] lr : mutex_lock+0x2c/0x50
[   56.286962] sp : ffff0000219af1b0
[   56.290264] x29: ffff0000219af1b0 x28: ffff800f64de49a0
[   56.295565] x27: 0000000000000000 x26: 0000000000000015
[   56.300865] x25: 0000000000000000 x24: 0000000000000000
[   56.306165] x23: 0000000000000000 x22: ffff000011117000
[   56.311465] x21: ffff800f64dfc080 x20: 0000000000000020
[   56.316766] x19: 0000000000000020 x18: 0000000000000001
[   56.322066] x17: 0000000000000000 x16: ffff800f2e077080
[   56.327367] x15: 0000000000000004 x14: 0000000000000000
[   56.332667] x13: ffff000010964438 x12: 0000000000000002
[   56.337967] x11: 0000000000000000 x10: 0000000000000c70
[   56.343268] x9 : ffff0000219af120 x8 : ffff800f2e077d50
[   56.348568] x7 : 0000000000000027 x6 : 000000062a9d6a84
[   56.353869] x5 : 0000000000000000 x4 : ffff800f2e077480
[   56.359169] x3 : 0000000000000008 x2 : ffff800f2e077080
[   56.364469] x1 : 0000000000000000 x0 : 0000000000000020
[   56.369770] Process ip (pid: 2034, stack limit = 0x00000000c862da3a)
[   56.376110] Call trace:
[   56.378546]  __ll_sc___cmpxchg_case_acq_64+0x4/0x20
[   56.383414]  drain_workqueue+0x34/0x198
[   56.387247]  nicvf_open+0x48/0x9e8 [nicvf]
[   56.391334]  nicvf_open+0x898/0x9e8 [nicvf]
[   56.395507]  nicvf_xdp+0x1bc/0x238 [nicvf]
[   56.399595]  dev_xdp_install+0x68/0x90
[   56.403333]  dev_change_xdp_fd+0xc8/0x240
[   56.407333]  do_setlink+0x8e0/0xbe8
[   56.410810]  __rtnl_newlink+0x5b8/0x6d8
[   56.414634]  rtnl_newlink+0x54/0x80
[   56.418112]  rtnetlink_rcv_msg+0x22c/0x2f8
[   56.422199]  netlink_rcv_skb+0x60/0x120
[   56.426023]  rtnetlink_rcv+0x28/0x38
[   56.429587]  netlink_unicast+0x1c8/0x258
[   56.433498]  netlink_sendmsg+0x1b4/0x350
[   56.437410]  sock_sendmsg+0x4c/0x68
[   56.440887]  ___sys_sendmsg+0x240/0x280
[   56.444711]  __sys_sendmsg+0x68/0xb0
[   56.448275]  __arm64_sys_sendmsg+0x2c/0x38
[   56.452361]  el0_svc_handler+0x9c/0x128
[   56.456186]  el0_svc+0x8/0xc
[   56.459056] Code: 35ffff91 2a1003e0 d65f03c0 f9800011 (c85ffc10)
[   56.465166] ---[ end trace 4a57fdc27b0a572c ]---
[   56.469772] Kernel panic - not syncing: Fatal exception

Fix it by checking nicvf_rx_mode_wq pointer in nicvf_open and nicvf_stop

Fixes: 2ecbe4f4 ("net: thunderx: replace global nicvf_rx_mode_wq work queue for all VFs to private for each of them")
Fixes: 2c632ad8 ("net: thunderx: move link state polling function to VF")
Reported-by: Matteo Croce <mcroce@redhat.com>
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Tested-by: Matteo Croce <mcroce@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>