iommu/arm-smmu: Protect against concurrent attach from different domains

Currently we're relying on the smmu_domain->lock for synchronizing
attach and detach.  This is a problem because each domain has its own
smmu_domain->lock, so if multiple different domains try to attach to the
same device at the same time, they'll be racing.

Fix the race by holding a lock that's part of the smmu
structure (attach_lock should do just fine).

The test case that uncovered this was:

    # cd /sys/kernel/debug/iommu/tests/soc:qcom,msm-audio-ion/
    # while :; do cat profiling; done &
    # while :; do cat profiling; done &

Change-Id: I8a60cdc214c91967aff63882e3a7280865ffda9e
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>