BACKPORT: Smack: Prevent the * and @ labels from being used in SMACK64EXEC

BACKPORT: Smack: Prevent the * and @ labels from being used in SMACK64EXEC

Smack prohibits processes from using the star ("*") and web ("@") labels
because we don't want files with those labels getting created implicitly.
All setting of those labels should be done explicitly. The trouble is that
there is no check for these labels in the processing of SMACK64EXEC. That
is repaired.

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

Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
(cherry-picked from upstream 19760ad03cc639d6f6f8e9beff0f8e6df654b677)