When attempt to run worker (ath10k_sta_rc_update_wk) after the station object
(ieee80211_sta) delete will trigger the kernel panic.

This problem arise in AP + Mesh configuration, Where the current node AP VAP
and neighbor node mesh VAP MAC address are same. When the current mesh node
try to establish the mesh link with neighbor node, driver peer creation for
the neighbor mesh node fails due to duplication MAC address. Already the AP
VAP created with same MAC address.

It is caused by the following scenario steps.

Steps:
1. In above condition, ath10k driver sta_state callback (ath10k_sta_state)
   fails to do the state change for a station from IEEE80211_STA_NOTEXIST
   to IEEE80211_STA_NONE due to peer creation fails. Sta_state callback is
   called from ieee80211_add_station() to handle the new station
   (neighbor mesh node) request from the wpa_supplicant.
2. Concurrently ath10k receive the sta_rc_update callback notification from
   the mesh_neighbour_update() to handle the beacon frames of the above
   neighbor mesh node. since its atomic callback, ath10k driver queue the
   work (ath10k_sta_rc_update_wk) to handle rc update.
3. Due to driver sta_state callback fails (step 1), mac80211 free the station
   object.
4. When the worker (ath10k_sta_rc_update_wk) scheduled to run, it will access
   the station object which is already deleted. so it will trigger kernel
   panic.

Added the peer exist check in sta_rc_update callback before queue the work.

Kernel Panic log:

Unable to handle kernel NULL pointer dereference at virtual address 00000000
pgd = c0204000
[00000000] *pgd=00000000
Internal error: Oops: 17 [#1] PREEMPT SMP ARM
CPU: 1 PID: 1833 Comm: kworker/u4:2 Not tainted 3.14.77 #1
task: dcef0000 ti: d72b6000 task.ti: d72b6000
PC is at pwq_activate_delayed_work+0x10/0x40
LR is at pwq_activate_delayed_work+0xc/0x40
pc : [<c023f988>]    lr : [<c023f984>]    psr: 40000193
sp : d72b7f18  ip : 0000007a  fp : d72b6000
r10: 00000000  r9 : dd404414  r8 : d8c31998
r7 : d72b6038  r6 : 00000004  r5 : d4907ec8  r4 : dcee1300
r3 : ffffffe0  r2 : 00000000  r1 : 00000001  r0 : 00000000
Flags: nZcv  IRQs off  FIQs on  Mode SVC_32  ISA ARM  Segment kernel
Control: 10c5787d  Table: 595bc06a  DAC: 00000015
...
Process kworker/u4:2 (pid: 1833, stack limit = 0xd72b6238)
Stack: (0xd72b7f18 to 0xd72b8000)
7f00:                                                       00000001 dcee1300
7f20: 00000001 c02410dc d8c31980 dd404400 dd404400 c0242790 d8c31980 00000089
7f40: 00000000 d93e1340 00000000 d8c31980 c0242568 00000000 00000000 00000000
7f60: 00000000 c02474dc 00000000 00000000 000000f8 d8c31980 00000000 00000000
7f80: d72b7f80 d72b7f80 00000000 00000000 d72b7f90 d72b7f90 d72b7fac d93e1340
7fa0: c0247404 00000000 00000000 c0208d20 00000000 00000000 00000000 00000000
7fc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
7fe0: 00000000 00000000 00000000 00000000 00000013 00000000 00000000 00000000
[<c023f988>] (pwq_activate_delayed_work) from [<c02410dc>] (pwq_dec_nr_in_flight+0x58/0xc4)
[<c02410dc>] (pwq_dec_nr_in_flight) from [<c0242790>] (worker_thread+0x228/0x360)
[<c0242790>] (worker_thread) from [<c02474dc>] (kthread+0xd8/0xec)
[<c02474dc>] (kthread) from [<c0208d20>] (ret_from_fork+0x14/0x34)
Code: e92d4038 e1a05000 ebffffbc[69210.619376] SMP: failed to stop secondary CPUs
Rebooting in 3 seconds..
Signed-off-by: Karthikeyan Periyasamy <periyasa@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>