f2fs: keep io_flags to avoid IO split due to different op_flags in two fio holders

Let's attach io_flags to bio only, so that we can merge IOs given original
io_flags only.

Fixes: 64bf0eef ("f2fs: pass the bio operation to bio_alloc_bioset")
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>