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

[jhb]

Summary

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.

[jhb]

I was able to verify that a character device using the default device pager still shows up with a device name in vmstat -o and procstat -v output.

[jhb]

More interesting would be for someone to fix some of the other drivers in the tree using cdev_pager_allocate() to add methods. I've added two simple examples in this series, but drm-kmod would probably be a more useful one to fix, perhaps by fixing the linuxkpi wrappers?

[kib]

sys/vm/device_pager.c

106