net: prevent setting ttl=0 via IP_TTL

A regression is introduced by the following commit:

	commit 4d52cfbef6
	Author: Eric Dumazet <eric.dumazet@gmail.com>
	Date:   Tue Jun 2 00:42:16 2009 -0700

	    net: ipv4/ip_sockglue.c cleanups

	    Pure cleanups

but it is not a pure cleanup...

	-               if (val != -1 && (val < 1 || val>255))
	+               if (val != -1 && (val < 0 || val > 255))

Since there is no reason provided to allow ttl=0, change it back.

Reported-by: nitin padalia <padalia.nitin@gmail.com>
Cc: nitin padalia <padalia.nitin@gmail.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_sockglue.c

@@ -590,7 +590,7 @@ static int do_ip_setsockopt(struct sock *sk, int level,
 	case IP_TTL:
 		if (optlen < 1)
 			goto e_inval;
-		if (val != -1 && (val < 0 || val > 255))
+		if (val != -1 && (val < 1 || val > 255))
 			goto e_inval;
 		inet->uc_ttl = val;
 		break;