Add the MEM_EXTRACT_PADDR ioctl to /dev/mem.

This allows privileged userspace processes to find information about the
physical page backing a given mapping. It is useful in applications
such as DPDK which perform some of their own memory management.

Reviewed by: kib, jhb (previous version)
MFC after: 2 weeks
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara Inc.
Differential Revision: https://reviews.freebsd.org/D26237