KVM: Move gfn_to_pfn_cache invalidation to invalidate_range_end hook

KVM: Move gfn_to_pfn_cache invalidation to invalidate_range_end hook

The existing retry loop in hva_to_pfn_retry() is extremely pessimistic.
If there are any concurrent invalidations running, it's effectively just
a complex busy wait loop because its local mmu_notifier_retry_cache()
function will always return true.

Since multiple invalidations can be running in parallel, this can result
in a situation where hva_to_pfn_retry() just backs off and keep retrying
for ever, not making any progress.

Solve this by being a bit more selective about when to retry.

Introduce a separate 'needs invalidation' flag to the GPC, which allows
it to be marked invalid even while hva_to_pfn_retry() has dropped the
lock to map the newly-found PFN. This allows the invalidation to moved
to the range_end hook, and the retry loop only occurs for a given GPC if
its particular uHVA is affected.

However, the contract for invalidate_range_{start,end} is not like a
simple TLB; the pages may have been freed by the time the end hook is
called. A "device" may not create new mappings after the _start_ hook is
called. To meet this requirement, hva_to_pfn_retry() now waits until no
invalidations are currently running which may affect its uHVA, before
finally setting the ->valid flag and returning.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>