f2fs: wake up gc thread immediately when gc_urgent is set

Fixes: 5b0e95398e2b ("f2fs: introduce sbi->gc_mode to determine the policy")
Signed-off-by: Sheng Yong <shengyong1@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>