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 openzfs#16808

include/os/freebsd/spl/sys/vnode.h

@@ -91,6 +91,7 @@ vn_is_readonly(vnode_t *vp)
 	((vp)->v_object != NULL && \
 	(vp)->v_object->resident_page_count > 0)
 
+#ifndef IN_BASE
 static __inline void
 vn_flush_cached_data(vnode_t *vp, boolean_t sync)
 {
@@ -101,6 +102,7 @@ vn_flush_cached_data(vnode_t *vp, boolean_t sync)
 		zfs_vmobject_wunlock(vp->v_object);
 	}
 }
+#endif
 
 #define	vn_exists(vp)		do { } while (0)
 #define	vn_invalid(vp)		do { } while (0)