FreeBSD: Return ifndef IN_BASE back to fix the build

FreeBSD's libprocstat seems to build kernel code in user space,
which does not work here due to undefined vnode_t.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Martin Matuska <mm@FreeBSD.org>
Signed-off-by:Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #16808