iommu/dma: Relax locking in iommu_dma_prepare_msi()

iommu/dma: Relax locking in iommu_dma_prepare_msi()

Since commit ece6e6f0218b ("iommu/dma-iommu: Split iommu_dma_map_msi_msg()
in two parts"), iommu_dma_prepare_msi() should no longer have to worry
about preempting itself, nor being called in atomic context at all. Thus
we can downgrade the IRQ-safe locking to a simple mutex to avoid angering
the new might_sleep() check in iommu_map().

Reported-by: Qian Cai <[email protected]>
Tested-by: Jean-Philippe Brucker <[email protected]>
Signed-off-by: Robin Murphy <[email protected]>
Signed-off-by: Joerg Roedel <[email protected]>