f2fs: add f2fs_init_write_merge_io function

Almost all other initialization of variables in f2fs_fill_super are
extraced to a single function. Also do it for write_io[], which can
make code more clean.

This patch just refactors the code, theres no functional change.

Signed-off-by: Yufen Yu <yuyufen@huawei.com>
Reviewed-by: Chao Yu <chao@kernel.org>
[Jaegeuk Kim: clean up]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>