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

commit d95e674c01cfb5461e8b9fdeebf6d878c9b80b2f upstream.

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>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/ceph/caps.c

@@ -1030,6 +1030,8 @@ 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);