x86/mm: {paging, sh}_{cmpxchg, write}_guest_entry() cannot fault

x86/mm: {paging, sh}_{cmpxchg, write}_guest_entry() cannot fault

As of 2d0557c5cbeb ("x86: Fold page_info lock into type_info") we
haven't been updating guest page table entries through linear page
tables anymore. All updates have been using domain mappings since then.
Drop the use of guest/user access helpers there, and hence also the
boolean return values of the involved functions.

update_intpte(), otoh, gets its boolean return type retained for now,
as we may want to bound the CMPXCHG retry loop, indicating failure to
the caller instead when the retry threshold got exceeded.

With this {,__}cmpxchg_user() become unused, so they too get dropped.
(In fact, dropping them was the motivation of making the change.)

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Tim Deegan <tim@xen.org>