nilfs2: remove nilfs_alloc_seg_bio

bio_alloc will never fail when it can sleep.  Remove the now simple
nilfs_alloc_seg_bio helper and open code it in the only caller.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220124091107.642561-3-hch@lst.de


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