pktcdvd: Switch to bio_kmalloc()

This is prep work for killing bi_destructor - previously, pktcdvd had
its own pkt_bio_alloc which was basically duplication bio_kmalloc(),
necessitating its own bi_destructor implementation.

v5: Un-reorder some functions, to make the patch easier to review

Signed-off-by: Kent Overstreet <koverstreet@google.com>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>