bio: get rid of bio_vec clearing

We don't need to clear the memory used for adding bio_vec entries,
since nobody should be looking at members unitialized. Any valid
use should be below bio->bi_vcnt, and that members up until that count
must be valid since they were added through bio_add_page().

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>