device_pager: Add cdev_pager_get_path to retrieve the "path" for an object

This wraps a new optional cdev_pg_path method in struct
cdev_pager_ops. If the method pointer is NULL, the function does
nothing. The old device pager reads the pathname of the cdev stored
in the object handle to match the existing code.

Retire the OBJ_CDEVH flag as it is no longer needed.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D49335