seccomp: add rule to reject umount -f

seccomp: add rule to reject umount -f

If a container has a bind mount from a host nfs or fuse
filesystem, and does 'umount -f', it will disconnect the
host's filesystem.  This patch adds a seccomp rule to
block umount -f from a container.  It also adds that rule
to the default seccomp profile.

Thanks stgraber for the idea :)

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>