Output a warning if we cannot bind to the specified interface.
https://bugzilla.redhat.com/show_bug.cgi?id=81640

--- iputils-ss021109-vanilla/ping.c	Thu Nov  7 23:53:21 2002
+++ iputils/ping.c	Sun Jan 12 03:39:24 2003
@@ -285,6 +285,9 @@
 						perror("ping: IP_MULTICAST_IF");
 						exit(2);
 					}
+				} else if (icmp_sock >= 0) {
+					/* We possible tried to SO_BINDTODEVICE() a subinterface like 'eth0:1' */
+					perror("Warning: cannot bind to specified iface, falling back");
 				}
 			}
 		}