nullfs traversing mount points (with working code)

Hi,

Recently I had the need to access nfs automounts (for example /net) from
within jails, I searched everywhere but couldn't find any suitable
solution. It seems we still don't support mount point traversal in nullfs,
even though many people have asked for it. I had no choice so I spent a bit
of time adding the support myself, which worked satisfactorily for my own
purpose. I wonder if anyone's interested in taking the code, polishing it
and eventually checking it in. The patch against 13.0-RELENG is at
https://people.freebsd.org/~luoqi/nullxmnt.diff

Best
-luoqi