uio segment is empty. In this case no dma segment is create by
bus_dmamap_load_buffer, but the calling routine clears the first flag.
Under certain combinations of addresses of the first and second mbuf/uio
buffer this leads to corrupted DMA segment descriptors. This was already
fixed by tmm in sparc64/sparc64/iommu.c.
PR: kern/47733
Reviewed by: sam
Approved by: jake (mentor)