KVM: MMU: simplify last_pte_bitmap

Branch-free code is fun and everybody knows how much Avi loves it,
but last_pte_bitmap takes it a bit to the extreme.  Since the code
is simply doing a range check, like

	(level == 1 ||
	 ((gpte & PT_PAGE_SIZE_MASK) && level < N)

we can make it branch-free without storing the entire truth table;
it is enough to cache N.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>