Protect it with the tlb_lock instead of using atomics.
The move puts it in or near the same cacheline as the lock;
using the lock means we don't need a second atomic operation
in order to perform the update.  Which makes it cheap to also
update pending_flush in tlb_flush_by_mmuidx_async_work.
Tested-by: NEmilio G. Cota <cota@braap.org>
Reviewed-by: NEmilio G. Cota <cota@braap.org>
Signed-off-by: NRichard Henderson <richard.henderson@linaro.org>