vm_object: do not assume that un_pager.devp.dev is cdev

It is subtype-specific handle. Mark OBJT_DEVICE that do fill cdev into
the handle, with a new object flag OBJ_CDEVH.

PR: 282533
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D47443