cdma: xilinx: Fix cdmatest driver crashing when unloading as a module

This patch fixes the issue cdmatest
driver crashing when unloading as a module.

get_task_struct and put_task_struct manage a thread's reference count.
After we done get_task_struct, it will always be safe to access that memory,
until we release it.If we call put_task_struct and
We are the last one with a reference count, it does a bunch of cleanup.

crash dump:
zynq>rmmod cdmatest
Unable to handle kernel NULL pointer dereference at virtual address 00000000
pgd = 7e398000
[00000000] *pgd=3e286831, *pte=00000000, *ppte=00000000
Internal error: Oops - BUG: 17 [#1] PREEMPT SMP ARM
Modules linked in: cdmatest(-)
CPU: 1 PID: 630 Comm: rmmod Not tainted 4.0.0-xilinx-25051-gb6960df #34
Hardware name: Xilinx Zynq Platform
task: 7e1f49c0 ti: 7e1ee000 task.ti: 7e1ee000
PC is at exit_creds+0x14/0x84
LR is at __put_task_struct+0x10/0x58
pc : [<400375d0>]    lr : [<4001ead4>]    psr: 60000013
sp : 7e1efec8  ip : 00000000  fp : 00200200
r10: 7e3705ac  r9 : 00100100  r8 : 7d25dd0c
r7 : 3f0016f0  r6 : 00000000  r5 : 7e13e648  r4 : 7e13e640
r3 : 00000000  r2 : 00000000  r1 : 60000013  r0 : 00000000
Flags: nZCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment user
Control: 18c5387d  Table: 3e39804a  DAC: 00000015
Process rmmod (pid: 630, stack limit = 0x7e1ee210)
Stack: (0x7e1efec8 to 0x7e1f0000)
fec0:                   7e13e640 4001ead4 7e13e640 40036448 7d25dd0c 7d25dd00
fee0: 7d25d580 3f0016f0 3f0016f0 3f0002f4 7e113000 7d25dd0c 00000000 7e113010
ff00: 3f001728 7e113044 00000081 4000df64 7e1ee000 00000000 00000000 40270458
ff20: 40270440 7e113010 3f001728 4026ece8 3f001728 7e113010 3f001728 4026f320
ff40: 3f001728 00000880 3ebf7f68 4026ea84 3f00176c 4006f2a4 7dc25880 616d6463
ff60: 74736574 45535500 00000020 00000000 7e1f49c0 7e1f4dc8 00000000 00000006
ff80: 7e1f49c0 7e1f4dc8 000f2040 40034da4 7e1ee000 001effb0 3ebf7f68 00000000
ffa0: 00000001 4000dde0 3ebf7f68 00000000 3ebf7f68 00000880 000fa0e0 00000000
ffc0: 3ebf7f68 00000000 00000001 00000081 00000001 00000000 3ebf7e8c 00000000
ffe0: 000fa0e0 3ebf7af0 0002cd74 36e5ba3c 60000010 3ebf7f68 00000000 00000000
[<400375d0>] (exit_creds) from [<4001ead4>] (__put_task_struct+0x10/0x58)
[<4001ead4>] (__put_task_struct) from [<40036448>] (kthread_stop+0x90/0x98)
[<40036448>] (kthread_stop) from [<3f0002f4>] (xilinx_cdmatest_remove+0x58/0xd0 [cdmatest])
[<3f0002f4>] (xilinx_cdmatest_remove [cdmatest]) from [<40270458>] (platform_drv_remove+0x18/0x30)
[<40270458>] (platform_drv_remove) from [<4026ece8>] (__device_release_driver+0x7c/0xc0)
[<4026ece8>] (__device_release_driver) from [<4026f320>] (driver_detach+0x84/0xac)
[<4026f320>] (driver_detach) from [<4026ea84>] (bus_remove_driver+0x64/0x8c)
[<4026ea84>] (bus_remove_driver) from [<4006f2a4>] (SyS_delete_module+0x154/0x1d0)
[<4006f2a4>] (SyS_delete_module) from [<4000dde0>] (ret_fast_syscall+0x0/0x34)
Code: e1a04000 e5903304 e3a02000 e5900300 (e5933000)
---[ end trace f700ab4d72219d3d ]---
Segmentation fault

Signed-off-by: Kedareswara rao Appana <appanad@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>