Revert "f2fs: reuse nids more aggressively"

Commit 26834466 ("f2fs: reuse nids more aggressively") tries to
reuse nids as many as possilbe, in order to mitigate producing obsolete
node pages in page cache.

But acutally, before we reuse the nids and related node page cache,
we will always invalidate that node page, so there will be not any
obsolete node pages in cache.

Let's just revert previous commit, so that nm_i::next_scan_nid can be
increased ascendingly, making __build_free_nids traverses all NAT pages
more easily, finally, free nid bitmap cache can be enabled as soon as
possible.

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