xen: lock pte pages while pinning/unpinning

When a pagetable is created, it is made globally visible in the rmap
prio tree before it is pinned via arch_dup_mmap(), and remains in the
rmap tree while it is unpinned with arch_exit_mmap().

This means that other CPUs may race with the pinning/unpinning
process, and see a pte between when it gets marked RO and actually
pinned, causing any pte updates to fail with write-protect faults.

As a result, all pte pages must be properly locked, and only unlocked
once the pinning/unpinning process has finished.

In order to avoid taking spinlocks for the whole pagetable - which may
overflow the PREEMPT_BITS portion of preempt counter - it locks and pins
each pte page individually, and then finally pins the whole pagetable.

Signed-off-by: Jeremy Fitzhardinge <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Hugh Dickens <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Keir Fraser <[email protected]>
Cc: Jan Beulich <[email protected]>

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