Add the MEM_EXTRACT_PADDR ioctl to /dev/mem.

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