The bio is always freed after running crypt_free_buffer_pages(), so it
isn't necessary to clear bv->bv_page.

Cc: Mike Snitzer <snitzer@redhat.com>
Cc:dm-devel@redhat.com
Signed-off-by: NMing Lei <ming.lei@redhat.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>