exec: Test the ptracer's saved cred to see if the tracee can gain caps

Now that we have user namespaces and non-global capabilities verify
the tracer has capabilities in the relevant user namespace instead
of in the current_user_ns().

As the test for setting LSM_UNSAFE_PTRACE_CAP is currently
ptracer_capable(p, current_user_ns()) and the new task credentials are
in current_user_ns() this change does not have any user visible change
and simply moves the test to where it is used, making the code easier
to read.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>