x86/mm: Eliminate hash table in sharing code as index of shared mfns

x86/mm: Eliminate hash table in sharing code as index of shared mfns

Eliminate the sharing hastable mechanism by storing a list head directly in the
page info for the case when the page is shared.  This does not add any extra
space to the page_info and serves to remove significant complexity from
sharing.

Signed-off-by: Adin Scannell <adin@scannell.ca>
Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Tim Deegan <tim@xen.org>