ipv4: Don't pre-seed hoplimit metric.

Always go through a new ip4_dst_hoplimit() helper, just like ipv6.

This allowed several simplifications:

1) The interim dst_metric_hoplimit() can go as it's no longer
   userd.

2) The sysctl_ip_default_ttl entry no longer needs to use
   ipv4_doint_and_flush, since the sysctl is not cached in
   routing cache metrics any longer.

3) ipv4_doint_and_flush no longer needs to be exported and
   therefore can be marked static.

When ipv4_doint_and_flush_strategy was removed some time ago,
the external declaration in ip.h was mistakenly left around
so kill that off too.

We have to move the sysctl_ip_default_ttl declaration into
ipv4's route cache definition header net/route.h, because
currently net/ip.h (where the declaration lives now) has
a back dependency on net/route.h

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
index 46eb3dc..eb68a0e 100644
--- a/net/ipv4/ip_gre.c
+++ b/net/ipv4/ip_gre.c
@@ -890,7 +890,7 @@
 			iph->ttl = ((struct ipv6hdr *)old_iph)->hop_limit;
 #endif
 		else
-			iph->ttl = dst_metric_hoplimit(&rt->dst);
+			iph->ttl = ip4_dst_hoplimit(&rt->dst);
 	}
 
 	((__be16 *)(iph + 1))[0] = tunnel->parms.o_flags;