KVM: x86/mmu: Refactor handling of forced 4k pages in page faults

Refactor the page fault handlers and mapping_level() to track the max
allowed page level instead of only tracking if a 4k page is mandatory
due to one restriction or another.  This paves the way for cleanly
consolidating tdp_page_fault() and nonpaging_page_fault(), and for
eliminating a redundant check on mmu_gfn_lpage_is_disallowed().

No functional change intended.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>