f2fs: fix macro definition on_f2fs_build_free_nids

The macro on_f2fs_build_free_nids accepts a parameter nmi,
but it was not used, rather the variable nm_i was directly used,
which may be a local variable inside a function that calls the macros.

Signed-off-by: Julian Sun <sunjunchao2870@gmail.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>