dm-buffered: fix bugs in __process_any_flush

There were these bugs in __process_any_flush:
* sync_writes only flushes bios with the REQ_SYNC flag. It should flush
  all write bios.
* If the I/O failed and flush succeeded, bio->bi_status would erroneously
  overwrite bio->bi_status with zero, and the I/O error would not be
  reported.
* We should only flush cache on write bios, the code would erroneously
  flush cache for read bios with REQ_SYNC or REQ_FUA too.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

---
 drivers/md/dm-buffered-target.c |   24 +++++++++++++++---------
 1 file changed, 15 insertions(+), 9 deletions(-)

Index: linux-2.6/drivers/md/dm-buffered-target.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-buffered-target.c
+++ linux-2.6/drivers/md/dm-buffered-target.c
@@ -256,19 +256,25 @@ static void _io(struct buffered_c *bc, s
 static blk_status_t __process_any_flush(struct buffered_c *bc, struct bio *bio)
 {
 	bool flush = false;
+	blk_status_t r = BLK_STS_OK;
 
-	if (bio->bi_opf & REQ_FUA) {
-		atomic_inc(&bc->stats[S_FUA]);
-		flush = true;
-	} else if (bc->sync_writes && (bio->bi_opf & REQ_SYNC)) {
-		atomic_inc(&bc->stats[S_SYNC_WRITES]);
-		flush = true;
+	if (bio_op(bio) == REQ_OP_WRITE) {
+		if (bio->bi_opf & REQ_FUA) {
+			atomic_inc(&bc->stats[S_FUA]);
+			flush = true;
+		} else if (bc->sync_writes) {
+			atomic_inc(&bc->stats[S_SYNC_WRITES]);
+			flush = true;
+		}
 	}
 
-	if (flush)
-		bio->bi_status = _buffered_flush(bc);
+	if (flush) {
+		r = _buffered_flush(bc);
+		if (r && !bio->bi_status)
+			bio->bi_status = r;
+	}
 
-	return bio->bi_status;
+	return r;
 }
 
 /*