[SCSI] libosd: osd_req_read_sg, optimize the single entry case

Since sg-read is a bidi operation, it is a gain to convert
a single sg entry into a regular read. Better do this in the
generic layer then force each caller to do so.

Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>