Smack: prevent revoke-subject from failing when unseen label is written to it

Special file /smack/revoke-subject will silently accept labels that are not
present on the subject label list. Nothing has to be done for such labels,
as there are no rules for them to revoke.

Targeted for git://git.gitorious.org/smack-next/kernel.git

Signed-off-by: Rafal Krypa <r.krypa@samsung.com>