[SCSI] sg: fix oops in the error path in sg_build_indirect()

When the allocation fails in sg_build_indirect(), an oops happens in
the error path. It's caused by an obvious typo.

Signed-off-by: Michal Schmidt <mschmidt@redhat.com>
Reported-by: Bob Tracy <rct@gherkin.frus.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Cc: Stable Tree <stable@kernel.org>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>