bcache: use REQ_PRIO to indicate bio for metadata

In cached_dev_cache_miss() and check_should_bypass(), REQ_META is used
to check whether a bio is for metadata request. REQ_META is used for
blktrace, the correct REQ_ flag should be REQ_PRIO. This flag means the
bio should be prior to other bio, and frequently be used to indicate
metadata io in file system code.

This patch replaces REQ_META with correct flag REQ_PRIO.

CC Adam Manzanares because he explains to me what REQ_PRIO is for.
Signed-off-by: Coly Li <colyli@suse.de>
Cc: Adam Manzanares <adam.manzanares@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/request.c

@@ -395,7 +395,7 @@ static bool check_should_bypass(struct cached_dev *dc, struct bio *bio)
 	 * unless the read-ahead request is for metadata (eg, for gfs2).
 	 */
 	if (bio->bi_opf & (REQ_RAHEAD|REQ_BACKGROUND) &&
-	    !(bio->bi_opf & REQ_META))
+	    !(bio->bi_opf & REQ_PRIO))
 		goto skip;
 
 	if (bio->bi_iter.bi_sector & (c->sb.block_size - 1) ||
@@ -877,7 +877,7 @@ static int cached_dev_cache_miss(struct btree *b, struct search *s,
 	}
 
 	if (!(bio->bi_opf & REQ_RAHEAD) &&
-	    !(bio->bi_opf & REQ_META) &&
+	    !(bio->bi_opf & REQ_PRIO) &&
 	    s->iop.c->gc_stats.in_use < CUTOFF_CACHE_READA)
 		reada = min_t(sector_t, dc->readahead >> 9,
 			      get_capacity(bio->bi_disk) - bio_end_sector(bio));