x86/ept: defer the invalidation until the p2m lock is released

x86/ept: defer the invalidation until the p2m lock is released

Holding the p2m lock while calling ept_sync_domain() is very expensive
since it does an on_selected_cpus() call.  IPIs on many socket
machines can be very slow and on_selected_cpus() is serialized.

It is safe to defer the invalidate until the p2m lock is released
except for two cases:

1. When freeing a page table page (since partial translations may be
   cached).
2. When reclaiming a zero page as part of PoD.

For these cases, add p2m_tlb_flush_sync() calls which will immediately
perform the invalidate before the page is freed or reclaimed.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: George Dunlap <geroge.dunlap@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>