iommu/io-pgtable-arm: Add quirk to quiet WARN_ON()

In situations where mapping/unmapping sequence can be controlled by
userspace, attempting to map over a region that has not yet been
unmapped is an error.  But not something that should spam dmesg.

Now that there is a quirk, we can also drop the selftest_running
flag, and use the quirk instead for selftests.

Acked-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Rob Clark <robdclark@chromium.org>
Link: https://lore.kernel.org/r/20250519175348.11924-6-robdclark@gmail.com


[will: Rename quirk to IO_PGTABLE_QUIRK_NO_WARN per Robin's suggestion]
Signed-off-by: Will Deacon <will@kernel.org>