Smack: change rule cap check

smk_write_change_rule() is calling capable rather than
the more correct smack_privileged(). This allows for setting
rules in violation of the onlycap facility. This is the
simple repair.

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



Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>