kern_rctl: Fix resource leak in error path

Ordinarily, rctl_write_outbuf frees 'sb'.  However, if we are in low memory
conditions we skip past the rctl_write_outbuf.  In that case, free 'sb'.

Reported by:	Coverity
CID:		1338539
Sponsored by:	EMC / Isilon Storage Division

sys/kern/kern_rctl.c

@@ -1866,6 +1866,7 @@ sys_rctl_get_limits(struct thread *td, struct rctl_get_limits_args *uap)
 	RCTL_RUNLOCK();
 	if (sbuf_error(sb) == ENOMEM) {
 		error = ERANGE;
+		sbuf_delete(sb);
 		goto out;
 	}