xfs: fix unsigned underflow in xfs_free_eofblocks

map_len is unsigned. Checking map_len <= 0 is buggy when it should be
below zero. So, check exact expression instead of map_len.
Signed-off-by: Kulikov Vasiliy <segooon@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/xfs_vnodeops.c

@@ -589,9 +589,9 @@ xfs_free_eofblocks(
 	 */
 	end_fsb = XFS_B_TO_FSB(mp, ((xfs_ufsize_t)ip->i_size));
 	last_fsb = XFS_B_TO_FSB(mp, (xfs_ufsize_t)XFS_MAXIOFFSET(mp));
-	map_len = last_fsb - end_fsb;
-	if (map_len <= 0)
+	if (last_fsb <= end_fsb)
 		return 0;
+	map_len = last_fsb - end_fsb;
 
 	nimaps = 1;
 	xfs_ilock(ip, XFS_ILOCK_SHARED);