ksmbd: use list_first_entry_or_null for opinfo_get_list()

The list_first_entry() macro never returns NULL.  If the list is
empty then it returns an invalid pointer.  Use list_first_entry_or_null()
to check if the list is empty.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/r/202505080231.7OXwq4Te-lkp@intel.com/
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: namjaejeon

fs/smb/server/oplock.c

@@ -146,12 +146,9 @@ static struct oplock_info *opinfo_get_list(struct ksmbd_inode *ci)
 {
 	struct oplock_info *opinfo;
 
-	if (list_empty(&ci->m_op_list))
-		return NULL;
-
 	down_read(&ci->m_lock);
-	opinfo = list_first_entry(&ci->m_op_list, struct oplock_info,
-					op_entry);
+	opinfo = list_first_entry_or_null(&ci->m_op_list, struct oplock_info,
+					  op_entry);
 	if (opinfo) {
 		if (opinfo->conn == NULL ||
 		    !atomic_inc_not_zero(&opinfo->refcount))