MFC 303753,308004: Add bounds checking on addresses used with /dev/mem.

303753:
Don't permit mappings of invalid physical addresses on amd64 via /dev/mem.

308004:
MFamd64: Add bounds checks on addresses used with /dev/mem.

Reject attempts to read from or memory map offsets in /dev/mem that are
beyond the maximum-supported physical address of the current CPU.