BACKPORT: Smack: Remove unnecessary smack_known_invalid

BACKPORT: Smack: Remove unnecessary smack_known_invalid

The invalid Smack label ("") and the Huh ("?") Smack label
serve the same purpose and having both is unnecessary.
While pulling out the invalid label it became clear that
the use of smack_from_secid() was inconsistent, so that
is repaired. The setting of inode labels to the invalid
label could never happen in a functional system, has
never been observed in the wild and is not what you'd
really want for a failure behavior in any case. That is
removed.

Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
(cherry-picked from upstream 152f91d4d11a30106b9cc0b27b47e0e80b633ee8)