f2fs: add has_enough_free_secs()

Replace !has_not_enough_free_secs w/ has_enough_free_secs.
BTW avoid nested 'if' statements in f2fs_balance_fs().

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