selinux,smack: fix subjective/objective credential use mixups

Jann Horn reported a problem with commit eb1231f73c4d ("selinux:
clarify task subjective and objective credentials") where some LSM
hooks were attempting to access the subjective credentials of a task
other than the current task.  Generally speaking, it is not safe to
access another task's subjective credentials and doing so can cause
a number of problems.

Further, while looking into the problem, I realized that Smack was
suffering from a similar problem brought about by a similar commit
1fb057dcde11 ("smack: differentiate between subjective and objective
task credentials").

This patch addresses this problem by restoring the use of the task's
objective credentials in those cases where the task is other than the
current executing task.  Not only does this resolve the problem
reported by Jann, it is arguably the correct thing to do in these
cases.

Cc: stable@vger.kernel.org
Fixes: eb1231f73c4d ("selinux: clarify task subjective and objective credentials")
Fixes: 1fb057dcde11 ("smack: differentiate between subjective and objective task credentials")
Reported-by: Jann Horn <jannh@google.com>
Acked-by: Eric W. Biederman <ebiederm@xmission.com>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>
