mrsas: Use mrsas_sge64 instead of iovec for the S/G list for passthru.

The DMA scatter/gather list for mrsas passthrough ioctl commands is
stored in a SGL at the end of the DCMD frame. Given the SGL member at
the end of the DCMD frame it seems likely this S/G list is formatted
as a fixed-width structure such as the type mrsas_sge64 and not as a
iovec which contains a kernel pointer and length that vary with the
native architecture size.

Reviewed by: imp
Obtained from: CheriBSD
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D40727