block: clear all of bi_opf in bio_set_op_attrs

Since commit 87374179

 ("block: add a proper block layer data direction
encoding") we only or the new op and flags into bi_opf in bio_set_op_attrs
instead of clearing the old value.  I've not seen any breakage with the
new behavior, but it seems dangerous.

Also convert it to an inline function to make the argument passing
safer.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>