[XFS] handle error returns from freeze_bdev

SGI-PV: 945483
SGI-Modid: xfs-linux-melb:xfs-kern:201884a

Signed-off-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Nathan Scott <[e-mail]>

fs/xfs/xfs_fsops.c

···
 		struct vfs *vfsp = XFS_MTOVFS(mp);
 		struct super_block *sb = freeze_bdev(vfsp->vfs_super->s_bdev);
 
-		if (sb) {
+		if (sb && !IS_ERR(sb)) {
 			xfs_force_shutdown(mp, XFS_FORCE_UMOUNT);
 			thaw_bdev(sb->s_bdev, sb);
 		}