x86/mm: Introduce lookup_address_in_mm()

x86/mm: Introduce lookup_address_in_mm()

Add a helper, lookup_address_in_mm(), to traverse the page tables of a
given mm struct.  KVM will use the helper to retrieve the host mapping
level, e.g. 4k vs. 2mb vs. 1gb, of a compound (or DAX-backed) page
without having to resort to implementation specific metadata.  E.g. KVM
currently uses different logic for HugeTLB vs. THP, and would add a
third variant for DAX-backed files.

Cc: Dan Williams <[email protected]>
Signed-off-by: Sean Christopherson <[email protected]>
Reviewed-by: Thomas Gleixner <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>