selftests/powerpc: Fix exit status of pkey tests

Since main() does not return a value explicitly, the
return values from FAIL_IF() conditions are ignored
and the tests can still pass irrespective of failures.
This makes sure that we always explicitly return the
correct test exit status.

Fixes: 1addb644 ("selftests/powerpc: Add test for execute-disabled pkeys")
Fixes: c27f2fd1

 ("selftests/powerpc: Add test for pkey siginfo verification")
Reported-by: Eirik Fuller <efuller@redhat.com>
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210118093145.10134-1-sandipan@linux.ibm.com