x86/mem_sharing: copy a page_lock version to be internal to memshr

x86/mem_sharing: copy a page_lock version to be internal to memshr

Patch cf4b30dca0a "Add debug code to detect illegal page_lock and put_page_type
ordering" added extra sanity checking to page_lock/page_unlock for debug builds
with the assumption that no hypervisor path ever locks two pages at once.

This assumption doesn't hold during memory sharing so we copy a version of
page_lock/unlock to be used exclusively in the memory sharing subsystem
without the sanity checks.

Signed-off-by: Tamas K Lengyel <tamas@tklengyel.com>
Acked-by: Jan Beulich <jbeulich@suse.com>