gre: Fix ttl inherit option

Specifying "... ttl inherit" currently does nothing on a GRE link
modify since the previous ttl value is retrieved up front. Fix this by
explicitly setting ttl to 0 when "inherit" is specified for the
option, since 0 represents the semantics of inherit.

Signed-off-by: Robert Shearman <rs823p@att.com>

ip/link_gre.c

@@ -276,7 +276,8 @@ get_failed:
 				if (uval > 255)
 					invarg("TTL must be <= 255\n", *argv);
 				ttl = uval;
-			}
+			} else
+				ttl = 0;
 		} else if (!matches(*argv, "tos") ||
 			   !matches(*argv, "tclass") ||
 			   !matches(*argv, "dsfield")) {