Smack: ptrace capability use fixes

This fixes a pair of problems in the Smack ptrace checks
related to checking capabilities. In both cases, as reported
by Lukasz Pawelczyk, the raw capability calls are used rather
than the Smack wrapper that check addition restrictions.
In one case, as reported by Jann Horn, the wrong task is being
checked for capabilities.

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