Implement vm_object_vnode() using vm_pager_getvp()

Allow vp_heldp argument to be NULL, in which case the returned vnode
is not held for tmpfs swap objects.

Reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D30070