[SCSI] scsi: add wrapper functions for sg buffer copy helper functions

LLDs need to copies data between the SG table in struct scsi_cmnd and
liner buffer. So they use the helper functions like

sg_copy_from_buffer(scsi_sglist(sc), scsi_sg_count(sc), buf, buflen)
sg_copy_to_buffer(scsi_sglist(sc), scsi_sg_count(sc), buf, buflen)

This patch just adds wrapper functions:

scsi_sg_copy_from_buffer(sc, buf, buflen)
scsi_sg_copy_to_buffer(sc, buf, buflen)

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>