f2fs: fix to check warm_data_age_threshold

hot_data_age_threshold is a non-zero positive number, and
condition 2 includes condition 1, so there is no need to
additionally judge whether t is 0. And let's remove it.

Signed-off-by: Yangtao Li <frank.li@vivo.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>