xen-blkback: bio_alloc can't fail if it is allow to sleep

Remove handling of NULL returns from sleeping bio_alloc calls given that
those can't fail.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220124091107.642561-13-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>