ipaccess: add bugtrag for the RSL link becomes up case

The RSL signal link becomes up for the ipaccess driver is tricky.
If the BSC forgets to use the E1 line used by OML for the RSL
link, we run into trouble.

This patch adds a bugtrap so people don't forget to appropriately
handle this case.
diff --git a/src/input/ipaccess.c b/src/input/ipaccess.c
index d0a1b4e..2acac89 100644
--- a/src/input/ipaccess.c
+++ b/src/input/ipaccess.c
@@ -333,6 +333,15 @@
 				ret = -EINVAL;
 				goto err;
 			}
+			/* this is a bugtrap, the BSC should be using the
+			 * virtual E1 line used by OML for this RSL link. */
+			if (sign_link->ts->line == line) {
+				LOGP(DINP, LOGL_ERROR,
+					"Fix your BSC, you should use the "
+					"E1 line used by the OML link for "
+					"your RSL link.\n");
+				return 0;
+			}
 			/* Finally, we know which OML link is associated with
 			 * this RSL link, attach it to this socket. */
 			bfd->data = new_line = sign_link->ts->line;