scsi: sg: Re-fix off by one in sg_fill_request_table()

Commit 109bade9 ("scsi: sg: use standard lists for sg_requests")
introduced an off-by-one error in sg_ioctl(), which was fixed by commit
bd46fc40 ("scsi: sg: off by one in sg_ioctl()").

Unfortunately commit 4759df90 ("scsi: sg: factor out
sg_fill_request_table()") moved that code, and reintroduced the
bug (perhaps due to a botched rebase).  Fix it again.

Fixes: 4759df90 ("scsi: sg: factor out sg_fill_request_table()")
Cc: stable@vger.kernel.org
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>