kernel - Fix efivar panic

kernel - Fix efivar panic

* The efirt driver uses direct-map mode, and creates an independent
  pmap to hold it.  The direct map typically uses userspace addresses,
  so SMAP must be disabled across any EFI call.

* Add smap_open(), smap_close(), smap_smep_disable(), and
  smap_smep_enable() calls for C.

  NOTE: We also have to set pcb_onfault to non-NULL to bypass failsafe
	checks done in trap().

* Hack the EFI address space code to disable SMAP and SMEP across
  the EFI call.  Unfortunately since we also have to disable SMEP,
  these functions have to manipulate CR4.

Reported-by: daftaupe