f2fs: use MAX_FREE_NIDS for the free nids target

F2FS has define MAX_FREE_NIDS for maximum of cached free nids target.

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>