staging/lustre/lloop: avoid panic during blockdev_info

Change the LL_IOC_LLOOP_INFO ioctl in the lustre lloop
device driver to return an error instead of causing
panics with LASSERT().
Signed-off-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-on: http://review.whamcloud.com/9888
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-4863Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/lustre/lustre/llite/lloop.c

@@ -624,7 +624,10 @@ static int lo_ioctl(struct block_device *bdev, fmode_t mode,
 	case LL_IOC_LLOOP_INFO: {
 		struct lu_fid fid;
 
-		LASSERT(lo->lo_backing_file != NULL);
+		if (lo->lo_backing_file == NULL) {
+			err = -ENOENT;
+			break;
+		}
 		if (inode == NULL)
 			inode = lo->lo_backing_file->f_dentry->d_inode;
 		if (lo->lo_state == LLOOP_BOUND)