selftests/seccomp: Test SECCOMP_IOCTL_NOTIF_ADDFD

Test whether we can add file descriptors in response to notifications.
This injects the file descriptors via notifications, and then uses kcmp
to determine whether or not it has been successful.

It also includes some basic sanity checking for arguments.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Chris Palmer <palmer@google.com>
Cc: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Jann Horn <jannh@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Robert Sesek <rsesek@google.com>
Cc: Tycho Andersen <tycho@tycho.ws>
Cc: Matt Denton <mpdenton@google.com>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Link: https://lore.kernel.org/r/20200603011044.7972-5-sargun@sargun.me


Co-developed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>