xfs: reduce metafile reservations

There is no point in reserving more space than actually available
on the data device for the worst case scenario that is unlikely to
happen.  Reserve at most 1/4th of the data device blocks, which is
still a heuristic.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

fs/xfs/libxfs/xfs_metafile.c

@@ -260,6 +260,7 @@ xfs_metafile_resv_init(
 	struct xfs_rtgroup	*rtg = NULL;
 	xfs_filblks_t		used = 0, target = 0;
 	xfs_filblks_t		hidden_space;
+	xfs_rfsblock_t		dblocks_avail = mp->m_sb.sb_dblocks / 4;
 	int			error = 0;
 
 	if (!xfs_has_metadir(mp))
@@ -297,6 +298,8 @@ xfs_metafile_resv_init(
 	 */
 	if (used > target)
 		target = used;
+	else if (target > dblocks_avail)
+		target = dblocks_avail;
 	hidden_space = target - used;
 
 	error = xfs_dec_fdblocks(mp, hidden_space, true);