[SCSI] fix write buffer length in scsi_req_map_sg()

sg's may have setup a the buffer with a different length than
the transfer length so we should be using the bufflen passed
in as the request's data len.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>