ksmbd: fix use-after-free by using call_rcu() for oplock_info

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

ksmbd: fix use-after-free by using call_rcu() for oplock_info

ksmbd currently frees oplock_info immediately using kfree(), even

though it is accessed under RCU read-side critical sections in places

like opinfo_get() and proc_show_files().

Since there is no RCU grace period delay between nullifying the pointer

and freeing the memory, a reader can still access oplock_info

structure after it has been freed. This can leads to a use-after-free

especially in opinfo_get() where atomic_inc_not_zero() is called on

already freed memory.

Fix this by switching to deferred freeing using call_rcu().