irqchip/gic-v3-its: Fix MSI alias accounting

The call to pci_for_each_dma_alias() in the ITS PCI code has aroused
suspicion in the past, and upon closer inspection does turn out to be
completely backwards. Rather than iterating through each RID alias of
the given device, what we actually want to be doing here is iterating
through all the *other* devices which may also alias the same RID, in
order to size the table for the worst case.

Do the right thing by ignoring the initial DMA aliases themselves and
just using that walk to detect an aliasing bridge, then walking back
down the bus topology as necessary to actually count everything else.

Our alias handling still isn't perfect, since we don't account for the
cases of certain bridges only taking ownership of transactions under
particular circumstances, but without completely reworking the ITS code
to cope with the notion of multiple DevIDs per device, it'll have to do.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

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