MFC 308820,308821: Fixes for fatal page faults on x86.

MFC 308820,308821: Fixes for fatal page faults on x86.

308820:
Report page faults due to reserved bits in PTEs as a separate fault type.

Rather than reporting a page fault due to a bad PTE as a protection
violation with the "rsv" flag, treat these faults as a separate type of
fault altogether.

308821:
MFamd64: Various fatal page fault fixes.

- If a page fault is triggered due to reserved bits in a PTE, treat it
  as a fatal fault and panic.
- If PG_NX is in use, report whether a fatal page fault is due to an
  instruction fetch or a data access.
- If a fatal page fault is due to reserved bits in a PTE, report that as
  the page fault type rather than a protection violation.

git-svn-id: svn://svn.freebsd.org/base/stable/10@310485 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f