block: Warn and free bio if bi_end_io is not set

In bio_endio if bio doesn't have bi_end_io (should be an error case),
we set bio to NULL and continue silently without freeing the bio. It
would be good to have a WARN and free the bio to avoid memory leak.

Signed-off-by: Muthukumar Ratty <muthur@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>