remove or annotate possibly-unused functions

remove or annotate possibly-unused functions

Clang notices more unused functions than GCC.

 * sh_next_page() is only used at GUEST_PAGING_LEVELS=2, so remove it from the
   other guest level translation units
 * rcu_batch_after() is completely unused.
 * Various of the COMPAT() generated functions are used only for their
   BUILD_BUG_ON() properties.  Annotate them all as __maybe_used.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>
Acked-by: Jan Beulich <jbeulich@suse.com>