f2fs: fix wrong continue condition in GC

We should decrease the frozen counter.

Cc: stable@vger.kernel.org
Fixes: 325163e9 ("f2fs: add gc_urgent_high_remaining sysfs node")
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>