block: Clean up special command handling logic

Remove special-casing of non-rw fs style requests (discard). The nomerge
flags are consolidated in blk_types.h, and rq_mergeable() and
bio_mergeable() have been modified to use them.

bio_is_rw() is used in place of bio_has_data() a few places. This is
done to to distinguish true reads and writes from other fs type requests
that carry a payload (e.g. write same).

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Acked-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/bio.h

@@ -386,9 +386,28 @@ static inline char *__bio_kmap_irq(struct bio *bio, unsigned short idx,
 /*
  * Check whether this bio carries any data or not. A NULL bio is allowed.
  */
-static inline int bio_has_data(struct bio *bio)
+static inline bool bio_has_data(struct bio *bio)
 {
-	return bio && bio->bi_io_vec != NULL;
+	if (bio && bio->bi_vcnt)
+		return true;
+
+	return false;
+}
+
+static inline bool bio_is_rw(struct bio *bio)
+{
+	if (!bio_has_data(bio))
+		return false;
+
+	return true;
+}
+
+static inline bool bio_mergeable(struct bio *bio)
+{
+	if (bio->bi_rw & REQ_NOMERGE_FLAGS)
+		return false;
+
+	return true;
 }
 
 /*