selftests/seccomp: Stop USER_NOTIF test if kcmp() fails

If kcmp() fails during the USER_NOTIF test, the test is likely to hang,
so switch from EXPECT to ASSERT.

Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Will Drewry <wad@chromium.org>
Cc: linux-kselftest@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Link: https://lore.kernel.org/r/20211103163039.2104830-2-keescook@chromium.org