Fix MNT_IGNORE for devfs, fdescfs and nullfs

The MNT_IGNORE flag can be used to mark certain filesystem mounts so
that utilities such as df(1) and mount(8) can filter out those mounts by
default. This can be used, for instance, to reduce the noise from
running container workloads inside jails which often have at least three
and sometimes as many as ten mounts per container.

The flag is supplied by the nmount(2) system call and is recorded so
that it can be reported by statfs(2). Unfortunately several filesystems
override the default behaviour and mask out the flag, defeating its
purpose. This change preserves the MNT_IGNORE flag for those filesystems
so that it can be reported correctly.

MFC after: 1 week