xen/arm: p2m: Invalidate the TLBs when write unlocking the p2m

xen/arm: p2m: Invalidate the TLBs when write unlocking the p2m

Sometimes the invalidation of the TLBs can be deferred until the p2m is
unlocked. This is for instance the case when multiple mappings are
removed. In other case, such as shattering a superpage, an immediate
flush is required.

Keep track whether a flush is needed directly in the p2m_domain structure
to allow serializing multiple changes. The TLBs will be invalidated when
write unlocking the p2m if necessary.

Also a new helper, p2m_flush_sync, has been introduced to force a
synchronous TLB invalidation.

Finally, replace the call to p2m_flush_tlb by p2m_flush_tlb_sync in
apply_p2m_changes.

Note this patch is not useful today, however follow-up patches will make
advantage of it.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Tested-by: Tamas K Lengyel <tamas@tklengyel.com>