When unloading the module 'g_hid.ko', the urb request will be dequeued and the
completion routine will be excuted. If there is no urb packet, the urb request
will not be added to the endpoint queue and the completion routine pointer in
urb request is NULL.

Accessing to this NULL function pointer will cause the Oops issue reported
below.

Add the code to check if the urb request is in the endpoint queue
or not. If the urb request is not in the endpoint queue, a negative
error code will be returned.

Here is the Oops log:

Unable to handle kernel NULL pointer dereference at virtual address 00000000
pgd = dedf0000
[00000000] *pgd=3ede5831, *pte=00000000, *ppte=00000000
Internal error: Oops: 80000007 [#1] ARM
Modules linked in: g_hid(-) usb_f_hid libcomposite
CPU: 0 PID: 923 Comm: rmmod Not tainted 3.18.0+ #2
Hardware name: Atmel SAMA5 (Device Tree)
task: df6b1100 ti: dedf6000 task.ti: dedf6000
PC is at 0x0
LR is at usb_gadget_giveback_request+0xc/0x10
pc : [<00000000>]    lr : [<c02ace88>]    psr: 60000093
sp : dedf7eb0  ip : df572634  fp : 00000000
r10: 00000000  r9 : df52e210  r8 : 60000013
r7 : df6a9858  r6 : df52e210  r5 : df6a9858  r4 : df572600
r3 : 00000000  r2 : ffffff98  r1 : df572600  r0 : df6a9868
Flags: nZCv  IRQs off  FIQs on  Mode SVC_32  ISA ARM  Segment user
Control: 10c53c7d  Table: 3edf0059  DAC: 00000015
Process rmmod (pid: 923, stack limit = 0xdedf6230)
Stack: (0xdedf7eb0 to 0xdedf8000)
7ea0:                                     00000000 c02adbbc df572580 deced608
7ec0: df572600 df6a9868 df572634 c02aed3c df577c00 c01b8608 00000000 df6be27c
7ee0: 00200200 00100100 bf0162f4 c000e544 dedf6000 00000000 00000000 bf010c00
7f00: bf0162cc bf00159c 00000000 df572980 df52e218 00000001 df5729b8 bf0031d0
[..]
[<c02ace88>] (usb_gadget_giveback_request) from [<c02adbbc>] (request_complete+0x64/0x88)
[<c02adbbc>] (request_complete) from [<c02aed3c>] (usba_ep_dequeue+0x70/0x128)
[<c02aed3c>] (usba_ep_dequeue) from [<bf010c00>] (hidg_unbind+0x50/0x7c [usb_f_hid])
[<bf010c00>] (hidg_unbind [usb_f_hid]) from [<bf00159c>] (remove_config.isra.6+0x98/0x9c [libcomposite])
[<bf00159c>] (remove_config.isra.6 [libcomposite]) from [<bf0031d0>] (__composite_unbind+0x34/0x98 [libcomposite])
[<bf0031d0>] (__composite_unbind [libcomposite]) from [<c02acee0>] (usb_gadget_remove_driver+0x50/0x78)
[<c02acee0>] (usb_gadget_remove_driver) from [<c02ad570>] (usb_gadget_unregister_driver+0x64/0x94)
[<c02ad570>] (usb_gadget_unregister_driver) from [<bf0160c0>] (hidg_cleanup+0x10/0x34 [g_hid])
[<bf0160c0>] (hidg_cleanup [g_hid]) from [<c0056748>] (SyS_delete_module+0x118/0x19c)
[<c0056748>] (SyS_delete_module) from [<c000e3c0>] (ret_fast_syscall+0x0/0x30)
Code: bad PC value
Signed-off-by: Songjun Wu <songjun.wu@atmel.com>
[nicolas.ferre@atmel.com: reworked the commit message]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Fixes: 914a3f3b ("USB: add atmel_usba_udc driver")
Cc: <stable@vger.kernel.org> # 2.6.x-ish
Signed-off-by: Felipe Balbi <balbi@ti.com>