selftests/seccomp: Zero out seccomp_notif

The seccomp_notif structure should be zeroed out prior to calling the
SECCOMP_IOCTL_NOTIF_RECV ioctl. Previously, the kernel did not check
whether these structures were zeroed out or not, so these worked.

This patch zeroes out the seccomp_notif data structure prior to calling
the ioctl.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>
Reviewed-by: Tycho Andersen <tycho@tycho.ws>
Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/r/20191229062451.9467-1-sargun@sargun.me
Fixes: 6a21cc50

 ("seccomp: add a return code to trap to userspace")
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>