asidalloc: Model PTEs and an asynchronous try_to_unmap_one() call

Following a kernel bug where try_to_unmap_one() may race with the ASID
roll-over and invalidate the VA corresponding to the unmapped PTE using
the old ASID, update the model to capture this scenario and the correct
kernel behaviour.

Reported-by: Will Deacon <will@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>