RDMA/mlx5: Fix implicit ODP hang on parent deregistration

In the Linux kernel, the following vulnerability has been resolved:

RDMA/mlx5: Fix implicit ODP hang on parent deregistration

Fix the destroy_unused_implicit_child_mr() to prevent hanging during
parent deregistration as of below [1].

Upon entering destroy_unused_implicit_child_mr(), the reference count
for the implicit MR parent is incremented using:
refcount_inc_not_zero().

A corresponding decrement must be performed if
free_implicit_child_mr_work() is not called.

The code has been updated to properly manage the reference count that
was incremented.

[1]
INFO: task python3:2157 blocked for more than 120 seconds.
Not tainted 6.12.0-rc7+ #1633
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:python3 state:D stack:0 pid:2157 tgid:2157 ppid:1685 flags:0x00000000
Call Trace:
<TASK>
__schedule+0x420/0xd30
schedule+0x47/0x130
__mlx5_ib_dereg_mr+0x379/0x5d0 [mlx5_ib]
? __pfx_autoremove_wake_function+0x10/0x10
ib_dereg_mr_user+0x5f/0x120 [ib_core]
? lock_release+0xc6/0x280
destroy_hw_idr_uobject+0x1d/0x60 [ib_uverbs]
uverbs_destroy_uobject+0x58/0x1d0 [ib_uverbs]
uobj_destroy+0x3f/0x70 [ib_uverbs]
ib_uverbs_cmd_verbs+0x3e4/0xbb0 [ib_uverbs]
? __pfx_uverbs_destroy_def_handler+0x10/0x10 [ib_uverbs]
? lock_acquire+0xc1/0x2f0
? ib_uverbs_ioctl+0xcb/0x170 [ib_uverbs]
? ib_uverbs_ioctl+0x116/0x170 [ib_uverbs]
? lock_release+0xc6/0x280
ib_uverbs_ioctl+0xe7/0x170 [ib_uverbs]
? ib_uverbs_ioctl+0xcb/0x170 [ib_uverbs]
__x64_sys_ioctl+0x1b0/0xa70
? kmem_cache_free+0x221/0x400
do_syscall_64+0x6b/0x140
entry_SYSCALL_64_after_hwframe+0x76/0x7e
RIP: 0033:0x7f20f21f017b
RSP: 002b:00007ffcfc4a77c8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007ffcfc4a78d8 RCX: 00007f20f21f017b
RDX: 00007ffcfc4a78c0 RSI: 00000000c0181b01 RDI: 0000000000000003
RBP: 00007ffcfc4a78a0 R08: 000056147d125190 R09: 00007f20f1f14c60
R10: 0000000000000001 R11: 0000000000000246 R12: 00007ffcfc4a7890
R13: 000000000000001c R14: 000056147d100fc0 R15: 00007f20e365c9d0
</TASK>