xfs: sanity-check the unused space before trying to use it

In xfs_dir2_data_use_free, we examine on-disk metadata and ASSERT if
it doesn't make sense.  Since a carefully crafted fuzzed image can cause
the kernel to crash after blowing a bunch of assertions, let's move
those checks into a validator function and rig everything up to return
EFSCORRUPTED to userspace.  Found by lastbit fuzzing ltail.bestcount via
xfs/391.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>

fs/xfs/libxfs/xfs_dir2_leaf.c

@@ -877,9 +877,13 @@ xfs_dir2_leaf_addname(
 	/*
 	 * Mark the initial part of our freespace in use for the new entry.
 	 */
-	xfs_dir2_data_use_free(args, dbp, dup,
-		(xfs_dir2_data_aoff_t)((char *)dup - (char *)hdr), length,
-		&needlog, &needscan);
+	error = xfs_dir2_data_use_free(args, dbp, dup,
+			(xfs_dir2_data_aoff_t)((char *)dup - (char *)hdr),
+			length, &needlog, &needscan);
+	if (error) {
+		xfs_trans_brelse(tp, lbp);
+		return error;
+	}
 	/*
 	 * Initialize our new entry (at last).
 	 */