FreeBSD: vn_flush_cached_data: observe vnode locking contract

vm_object_page_clean() expects that the associated vnode is locked
as VOP_PUTPAGES() may get called on the vnode.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Andriy Gapon <avg@FreeBSD.org>
Closes #14079