f2fs: blk_finish_plug of submit_bio in lfs mode

Expand the blk_finish_plug action from blkzoned to normal lfs mode,
since plug will cause the out-of-order IO submission, which is not
friendly to flash in lfs mode.

Signed-off-by: Yunlong Song <yunlong.song@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>