signal: Convert to the new fallthrough notation

Switch from using the /* fall through */ comment style notation to the new,
preferred notation as outlined in our docs.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
[christian.brauner@ubuntu.com: rewrite commit message]
Link: https://lore.kernel.org/r/20200814083932.4975-1-linmiaohe@huawei.com


Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>