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/fs/udf/directory.c b/fs/udf/directory.c
index 71f3e0b..988d535 100644
--- a/fs/udf/directory.c
+++ b/fs/udf/directory.c
@@ -87,7 +87,7 @@
 					brelse(tmp);
 			}
 			if (num) {
-				ll_rw_block(REQ_OP_READ, READA, num, bha);
+				ll_rw_block(REQ_OP_READ, REQ_RAHEAD, num, bha);
 				for (i = 0; i < num; i++)
 					brelse(bha[i]);
 			}