block/loop: implement REQ_FLUSH/FUA support

Deprecate REQ_HARDBARRIER and implement REQ_FLUSH/FUA instead.  Also,
instead of checking file->f_op->fsync() directly, look at the value of
vfs_fsync() and ignore -EINVAL return.
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

drivers/block/loop.c

@@ -477,17 +477,17 @@ static int do_bio_filebacked(struct loop_device *lo, struct bio *bio)
 	pos = ((loff_t) bio->bi_sector << 9) + lo->lo_offset;
 
 	if (bio_rw(bio) == WRITE) {
-		bool barrier = (bio->bi_rw & REQ_HARDBARRIER);
 		struct file *file = lo->lo_backing_file;
 
-		if (barrier) {
-			if (unlikely(!file->f_op->fsync)) {
+		/* REQ_HARDBARRIER is deprecated */
+		if (bio->bi_rw & REQ_HARDBARRIER) {
 			ret = -EOPNOTSUPP;
 			goto out;
 		}
 
+		if (bio->bi_rw & REQ_FLUSH) {
 			ret = vfs_fsync(file, 0);
-			if (unlikely(ret)) {
+			if (unlikely(ret && ret != -EINVAL)) {
 				ret = -EIO;
 				goto out;
 			}
@@ -495,9 +495,9 @@ static int do_bio_filebacked(struct loop_device *lo, struct bio *bio)
 
 		ret = lo_send(lo, bio, pos);
 
-		if (barrier && !ret) {
+		if ((bio->bi_rw & REQ_FUA) && !ret) {
 			ret = vfs_fsync(file, 0);
-			if (unlikely(ret))
+			if (unlikely(ret && ret != -EINVAL))
 				ret = -EIO;
 		}
 	} else