FreeBSD: fix world build after de198f2d9

The inline function vn_flush_cached_data() in vnode.h
must not be compiled when building BASE.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: Martin Matuska <mm@FreeBSD.org>
Closes #12743