f2fs: Allocate and stat mem used by free nid bitmap more accurately

This patch used f2fs_bitmap_size macro to calculate mem used by
free nid bitmap, and stat used mem including aligned part.

Signed-off-by: Yunlei He <heyunlei@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>