x86/pci: Comment pci_mmconfig_insert() obscure MCFG dependency

In pci_mmconfig_insert(), there's no reference to "addr" between locking
pci_mmcfg_lock and testing "addr", so it *looks* like we should move the
test before the lock.

But 07f9b61c ("x86/PCI: MMCONFIG: Check earlier for MMCONFIG region at
address zero") did that, which broke things by returning -EINVAL when
"addr" is zero instead of -EEXIST.

So 07f9b61c was reverted by 67d470e0 ("Revert "x86/PCI: MMCONFIG:
Check earlier for MMCONFIG region at address zero"").

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Link: https://lore.kernel.org/r/20231121183643.249006-8-helgaas@kernel.org


Tested-by: Tomasz Pala <gotar@polanet.pl>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>