Remove buf->b_saveaddr, assert that vmapbuf() is only called on pbuf's. Pass

Remove buf->b_saveaddr, assert that vmapbuf() is only called on pbuf's.  Pass
the user pointer and length to vmapbuf() rather then having it try to pull
the information out of the buffer.  vmapbuf() is now responsible for setting
b_data, b_bufsize, and b_bcount.

Also fix a bug in cam_periph_mapmem().  The procedure was failing to unmap
earlier vmapped bufs if later vmapbuf() calls in the loop failed.