ext4: do not send discards as barriers

ext4 already uses synchronous discards, no need to add I/O barriers.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

fs/ext4/mballoc.c

@@ -2567,7 +2567,7 @@ static inline void ext4_issue_discard(struct super_block *sb,
 	trace_ext4_discard_blocks(sb,
 			(unsigned long long) discard_block, count);
 	ret = sb_issue_discard(sb, discard_block, count, GFP_NOFS,
-			       BLKDEV_IFL_WAIT | BLKDEV_IFL_BARRIER);
+			       BLKDEV_IFL_WAIT);
 	if (ret == EOPNOTSUPP) {
 		ext4_warning(sb, "discard not supported, disabling");
 		clear_opt(EXT4_SB(sb)->s_mount_opt, DISCARD);