quota: make Q_XQUOTASYNC a noop

Now that XFS takes quota reservations into account there is no need to flush
anything before reporting quotas - in addition to beeing fully transactional
all quota information is also 100% coherent with the rest of the filesystem
now.

Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ben Myers <bpm@sgi.com>

fs/quota/quota.c

@@ -282,10 +282,9 @@ static int do_quotactl(struct super_block *sb, int type, int cmd, qid_t id,
 	case Q_XGETQUOTA:
 		return quota_getxquota(sb, type, id, addr);
 	case Q_XQUOTASYNC:
-		/* caller already holds s_umount */
 		if (sb->s_flags & MS_RDONLY)
 			return -EROFS;
-		writeback_inodes_sb(sb, WB_REASON_SYNC);
+		/* XFS quotas are fully coherent now, making this call a noop */
 		return 0;
 	default:
 		return -EINVAL;