ceph: clear inode pointer when snap realm gets dropped by its inode

snap realm and corresponding inode have pointers to each other.
The two pointer should get clear at the same time. Otherwise,
snap realm's pointer may reference freed inode.

Cc: stable@vger.kernel.org # 4.17+
Signed-off-by: N"Yan, Zheng" <zyan@redhat.com>
Reviewed-by: NLuis Henriques <lhenriques@suse.com>
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>

fs/ceph/caps.c

@@ -1035,6 +1035,8 @@ static void drop_inode_snap_realm(struct ceph_inode_info *ci)
 	list_del_init(&ci->i_snap_realm_item);
 	ci->i_snap_realm_counter++;
 	ci->i_snap_realm = NULL;
+	if (realm->ino == ci->i_vino.ino)
+		realm->inode = NULL;
 	spin_unlock(&realm->inodes_with_caps_lock);
 	ceph_put_snap_realm(ceph_sb_to_client(ci->vfs_inode.i_sb)->mdsc,
 			    realm);