ipv4: raise IP_MAX_MTU to theoretical limit

As discussed last year [1], there is no compelling reason
to limit IPv4 MTU to 0xFFF0, while real limit is 0xFFFF

[1] : http://marc.info/?l=linux-netdev&m=135607247609434&w=2

Willem raised this issue again because some of our internal
regression tests broke after lo mtu being set to 65536.

IP_MTU reports 0xFFF0, and the test attempts to send a RAW datagram of
mtu + 1 bytes, expecting the send() to fail, but it does not.

Alexey raised interesting points about TCP MSS, that should be addressed
in follow-up patches in TCP stack if needed, as someone could also set
an odd mtu anyway.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Cc: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -112,7 +112,8 @@
 #define RT_FL_TOS(oldflp4) \
 	((oldflp4)->flowi4_tos & (IPTOS_RT_MASK | RTO_ONLINK))
 
-#define IP_MAX_MTU	0xFFF0
+/* IPv4 datagram length is stored into 16bit field (tot_len) */
+#define IP_MAX_MTU	0xFFFF
 
 #define RT_GC_TIMEOUT (300*HZ)
 
@@ -1227,10 +1228,7 @@ static unsigned int ipv4_mtu(const struct dst_entry *dst)
 			mtu = 576;
 	}
 
-	if (mtu > IP_MAX_MTU)
-		mtu = IP_MAX_MTU;
-
-	return mtu;
+	return min_t(unsigned int, mtu, IP_MAX_MTU);
 }
 
 static struct fib_nh_exception *find_exception(struct fib_nh *nh, __be32 daddr)