xfs: ratelimit xfs_discard_page messages

Use printk_ratelimit() to limit the amount of messages printed from
xfs_discard_page.  Without that a failing device causes a large
number of errors that doesn't really help debugging the underling
issue.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/xfs/xfs_aops.c

@@ -539,7 +539,7 @@ xfs_discard_page(
 	if (XFS_FORCED_SHUTDOWN(mp))
 		goto out_invalidate;
 
-	xfs_alert(mp,
+	xfs_alert_ratelimited(mp,
 		"page discard on page "PTR_FMT", inode 0x%llx, offset %llu.",
 			page, ip->i_ino, offset);