f2fs: move all the bio initialization into __bio_alloc

Move all the bio initialization into __bio_alloc, and some minor cleanups are
also added.

v3:
  Use 'bool' rather than 'int' as Kim suggested.

v2:
  Use 'is_read' rather than 'rw' as Yu Chao suggested.
  Remove the needless initialization of bio->bi_private.

Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>