Don't blatt first element of prv in sg_chain()

I realize that sg chaining is a ploy to make the rest of the kernel
devs feel the pain of the SCSI subsystem.  But this was a little
unsubtle.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>