RFC: switch the remaining architectures to use generic GUP v2

[Christoph Hellwig]

Hi Linus and maintainers,

below is a series to switch mips, sh and sparc64 to use the generic
GUP code so that we only have one codebase to touch for further
improvements to this code.  I don't have hardware for any of these
architectures, and generally no clue about their page table
management, so handle with care.

Changes since v1:
 - fix various issues found by the build bot
 - cherry pick and use the untagged_addr helper form Andrey
 - add various refactoring patches to share more code over architectures
 - move the powerpc hugepd code to mm/gup.c and sync it with the generic
   hup semantics