x86/mm: Add per-page locking for memory sharing, when audits are disabled

x86/mm: Add per-page locking for memory sharing, when audits are disabled

With the removal of the hash table, all that is needed now is locking
of individual shared pages, as new (gfn,domain) pairs are removed or
added from the list of mappings.

We recycle PGT_locked and use it to lock individual pages. We ensure deadlock
is averted by locking pages in increasing order.

The global lock remains for the benefit of the auditing code, and is
thus enabled only as a compile-time option.

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