x86/mm/sharing: Clean ups for relinquishing shared pages on destroy

x86/mm/sharing: Clean ups for relinquishing shared pages on destroy

When a domain is destroyed, its pages are freed in relinquish_resources in a
preemptible mode, in the context of a synchronous domctl.

P2m entries pointing to shared pages are, however, released during p2m cleanup
in an RCU callback, and in non-preemptible mode.

This is an O(n) operation for a very large n, which may include actually
freeing shared pages for which the domain is the last holder.

To improve responsiveness, move this operation to the preemtible portion of
domain destruction, during the synchronous domain_kill hypercall. And remove
the bulk of the work from the RCU callback.

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