[SCSI] libosd: write/read_sg_kern API

This is a trivial addition to the SG API that can receive kernel
pointers. It is only used by the out-of-tree test module. So
it's immediate need is questionable. For maintenance ease it might
just get in, as it's very small.

John.
do you need this in the Kernel, or is it only for osd_ktest.ko?

Signed-off-by: John A. Chandy <john.chandy@uconn.edu>
Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>