f2fs: clear discard_wake earlier

If SBI_NEED_FSCK is set, discard_wake will never be cleared. As a
result, the condition of wait_event_interruptible_timeout() is always
true, which gets discard thread run too frequently.

Signed-off-by: Sheng Yong <shengyong1@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>