Fix iommu_dvmamap_sync(): it was still operating as if the BUS_DMASYNC_* constants where flag bits (as in NetBSD), although they are consecutively numbered in FreeBSD. This would cause unnecessary flushing in the BUS_DMASYNC_POSTWRITE case, but was otherwise mostly harmless.
Fix iommu_dvmamap_sync(): it was still operating as if the BUS_DMASYNC_*
constants where flag bits (as in NetBSD), although they are consecutively
numbered in FreeBSD. This would cause unnecessary flushing in the
BUS_DMASYNC_POSTWRITE case, but was otherwise mostly harmless.

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