f2fs: rewrite f2fs_bio_alloc to make it simpler

Since, GFP_NOFS(__GFP_WAIT) is used for allocation requests of bio in f2fs.
So, there is no chance of returning NULL from the BIO allocation.

Making the bio allocation routine for f2fs simpler.

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Amit Sahrawat <a.sahrawat@samsung.com>