f2fs: fix not to allocate max_nid

The build_free_nid should not add free nids over nm_i->max_nid.
But, there was a hole that invalid free nid was added by the following scenario.

Let's suppose nm_i->max_nid = 150 and the last NAT page has 100 ~ 200 nids.

build_free_nids
  - get_current_nat_page loads the last NAT page
  - scan_nat_page can add 100 ~ 200 nids
    -> Bug here!
So, when scanning an NAT page, we should check each candidate whether it is
over max_nid or not.

Reviewed-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>