Don't cross mount boundaries when cleaning tmp files.

Mounting something in /tmp such as a build jail with nullfs mounts for
some directories can result in very surprising results the next day.

MFC after:	2 weeks
Relnotes:	yes


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271321 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f