f2fs: Use DIV_ROUND_UP() instead of open-coding

Replace the open-coded divisions with round-up by calls to the
DIV_ROUND_UP() helper macro.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>