f2fs: drop duplicate radix tree lookup of nat_entry_set

The nat entry is listed from the set list for freeing,
it's duplicate to do radix tree lookup again.

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
[Jaegeuk Kim: remove unnecessary f2fs_bug_on]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>