selftests: sigaltstack: do not use ksft_exit_skip after ksft_set_plan

Calling ksft_exit_skip after ksft_set_plan results in executing fewer tests
than planned.  Use ksft_test_result_skip when possible, or just bail out if
memory corruption is detected.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>