f2fs: switch discard_policy.timeout to bool type

While checking discard timeout, we use specified type
UMOUNT_DISCARD_TIMEOUT, so just replace doplicy.timeout with
it, and switch doplicy.timeout to bool type.

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