f2fs: don't skip checkpoint if there is no dirty node pages

This is the errorneous scenario.
1. write data
2. do checkpoint
3. produce some dirty node pages by the gc thread
4. write back dirty node pages
5. f2fs_put_super will skip the checkpoint, since dirty count for node pages is
  zero.

This patch removes such the wrong condition check.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>