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

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().

Fixes: 18b4fac5ef17 ("ksmbd: fix use-after-free in smb_break_all_levII_oplock()")
Cc: stable@vger.kernel.org
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>