KVM: x86/mmu: Refactor THP adjust to prep for changing query

Refactor transparent_hugepage_adjust() in preparation for walking the
host page tables to identify hugepage mappings, initially for THP pages,
and eventualy for HugeTLB and DAX-backed pages as well.  The latter
cases support 1gb pages, i.e. the adjustment logic needs access to the
max allowed level.

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