ceph: flush dirty inodes before proceeding with remount

xfstest generic/452 was triggering a "Busy inodes after umount" warning.
ceph was allowing the mount to go read-only without first flushing out
dirty inodes in the cache. Ensure we sync out the filesystem before
allowing a remount to proceed.

Cc: stable@vger.kernel.org
Link: http://tracker.ceph.com/issues/39571
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/ceph/super.c

@@ -845,13 +845,20 @@ static void ceph_umount_begin(struct super_block *sb)
 	return;
 }
 
+static int ceph_remount(struct super_block *sb, int *flags, char *data)
+{
+	sync_filesystem(sb);
+	return 0;
+}
+
 static const struct super_operations ceph_super_ops = {
 	.alloc_inode	= ceph_alloc_inode,
 	.destroy_inode	= ceph_destroy_inode,
 	.write_inode    = ceph_write_inode,
 	.drop_inode	= ceph_drop_inode,
 	.sync_fs        = ceph_sync_fs,
 	.put_super	= ceph_put_super,
+	.remount_fs	= ceph_remount,
 	.show_options   = ceph_show_options,
 	.statfs		= ceph_statfs,
 	.umount_begin   = ceph_umount_begin,