block: get rid of bio_rw and READA

These two are confusing leftover of the old world order, combining
values of the REQ_OP_ and REQ_ namespaces.  For callers that don't
special case we mostly just replace bi_rw with bio_data_dir or
op_is_write, except for the few cases where a switch over the REQ_OP_
values makes more sense.  Any check for READA is replaced with an
explicit check for REQ_RAHEAD.  Also remove the READA alias for
REQ_RAHEAD.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index 41d9c31..4e6da44 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -1105,7 +1105,7 @@
 	bitmap = mddev->bitmap;
 
 	/*
-	 * make_request() can abort the operation when READA is being
+	 * make_request() can abort the operation when read-ahead is being
 	 * used and no empty request is available.
 	 *
 	 */