Revert most of r360179.

I had failed to notice that sgsendccb() was using cam_periph_mapmem()
and thus was not passing down user pointers directly to drivers. In
practice this broke requests submitted from userland.

PR: 249395
Reported by: Trenton Schulz <trueos@norwegianrockcat.com>
Reviewed by: scottl
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D26550