f2fs: remove unnecessary null checking

When __GFP_DIRECT_RECLAIM (included in both GFP_NOIO and GFP_KERNEL) is
specified, bio_alloc_bioset() never fails to allocate a bio.
Commit 67883ade ("f2fs: remove FAULT_ALLOC_BIO") replaced
f2fs_bio_alloc() with bio_alloc_bioset(), but null checking after
bio_alloc_bioset() was still left.

Fixes: 67883ade ("f2fs: remove FAULT_ALLOC_BIO")
Signed-off-by: Kohei Enju <enjuk@amazon.com>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>