The idea here is just to give a demonstration of how one could safely use
the SECCOMP_RET_USER_NOTIF feature to do mount policies. This particular
policy is (as noted in the comment) not very interesting, but it serves to
illustrate how one might apply a policy dodging the various TOCTOU issues.

Signed-off-by: Tycho Andersen
CC: Kees Cook
CC: Andy Lutomirski
CC: Oleg Nesterov
CC: Eric W. Biederman
CC: "Serge E. Hallyn"
CC: Christian Brauner
CC: Tyler Hicks
CC: Akihiro Suda
Signed-off-by: Kees Cook