x86/mem_sharing: Fix RANDCONFIG build

x86/mem_sharing: Fix RANDCONFIG build

Travis reports: https://travis-ci.org/andyhhp/xen/jobs/633751811

  mem_sharing.c:361:13: error: 'rmap_has_entries' defined but not used [-Werror=unused-function]
   static bool rmap_has_entries(const struct page_info *page)
               ^
  cc1: all warnings being treated as errors

This happens in a release build (disables MEM_SHARING_AUDIT) when
CONFIG_MEM_SHARING is enabled.

Expand both trivial helpers into their single callsite.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Tamas K Lengyel <tamas@tklengyel.com>