[writeboost] add destroy_work_on_stack in dm_safe_io_internal

destroy_work_on_stack must be pair of INIT_WORK_ONSTACK

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>

----

I didn't know that INIT_WORK_ONSTACK must pair with this function

----

Even after this fix, redzone overwritten error is seen

```
[   95.050364] XFS (dm-7): Mounting Filesystem
[  100.904965] XFS (dm-7): Ending clean mount
[  116.008335] =============================================================================
[  116.008820] BUG kmalloc-4096 (Not tainted): Redzone overwritten
[  116.009192] -----------------------------------------------------------------------------
[  116.009192]
[  116.009694] Disabling lock debugging due to kernel taint
[  116.009698] INFO: 0xffff88002fd577b0-0xffff88002fd577b7. First byte 0x0 instead of 0xcc
[  116.010172] INFO: Freed in seq_release+0x1d/0x30 age=144 cpu=1 pid=3600
[  116.010578]  __slab_free+0x3d/0x2eb
[  116.010864]  kfree+0x1ad/0x1f0
[  116.011136]  seq_release+0x1d/0x30
[  116.011421]  close_pdeo+0x48/0xb0
[  116.011703]  proc_reg_release+0x5b/0x70
[  116.011996]  __fput+0xf5/0x250
[  116.012107]  ____fput+0xe/0x10
[  116.012222]  task_work_run+0xb5/0xe0
[  116.012345]  do_notify_resume+0x59/0x80
[  116.012468]  int_signal+0x12/0x17
[  116.012582] INFO: Slab 0xffffea0000bf5400 objects=7 used=6 fp=0xffff88002fd50000 flags=0x100000000004081
[  116.012990] INFO: Object 0xffff88002fd567b0 @offset=26544 fp=0x          (null)
```

but writeboost can finish the test. Is this bug really writeboost's?

It looks like this https://lkml.org/lkml/2013/2/28/438