Fix a race in pmap_emulate_accessed_dirty() that could trigger a EPT
misconfiguration VM-exit.

An EPT misconfiguration is triggered when the processor encounters a PTE
that is writable but not readable (WR=10). On processors that require A/D
bit emulation PG_M and PG_A map to EPT_PG_WRITE and EPT_PG_READ respectively.

If the PTE is updated as in the following code snippet:
	*pte |= PG_M;
	*pte |= PG_A;
then it is possible for another processor to observe the PTE after the PG_M
(aka EPT_PG_WRITE) bit is set but before PG_A (aka EPT_PG_READ) bit is set.

This will trigger an EPT misconfiguration VM-exit on the other processor.

Reported by:	rodrigc
Reviewed by:	grehan
MFC after:	3 days