f2fs: wake up discard_thread iff there is a candidate

commit 01983c715ad0e78842a885f361ad927a3a985994 upstream.

This patch fixes to avoid needless wake ups.

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