net: fix building errors on powerpc when CONFIG_RETPOLINE is not set

This commit fixes the errores reported when building for powerpc:

 ERROR: modpost: "ip6_dst_check" [vmlinux] is a static EXPORT_SYMBOL
 ERROR: modpost: "ipv4_dst_check" [vmlinux] is a static EXPORT_SYMBOL
 ERROR: modpost: "ipv4_mtu" [vmlinux] is a static EXPORT_SYMBOL
 ERROR: modpost: "ip6_mtu" [vmlinux] is a static EXPORT_SYMBOL

Fixes: f67fbeae ("net: use indirect call helpers for dst_mtu")
Fixes: bbd807df ("net: indirect call helpers for ipv4/ipv6 dst_check functions")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Brian Vazquez <brianvv@google.com>
Link: https://lore.kernel.org/r/20210204181839.558951-2-brianvv@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/route.c

@@ -1206,7 +1206,7 @@ INDIRECT_CALLABLE_SCOPE struct dst_entry *ipv4_dst_check(struct dst_entry *dst,
 		return NULL;
 	return dst;
 }
-EXPORT_SYMBOL(ipv4_dst_check);
+EXPORT_INDIRECT_CALLABLE(ipv4_dst_check);
 
 static void ipv4_send_dest_unreach(struct sk_buff *skb)
 {
@@ -1337,7 +1337,7 @@ INDIRECT_CALLABLE_SCOPE unsigned int ipv4_mtu(const struct dst_entry *dst)
 
 	return mtu - lwtunnel_headroom(dst->lwtstate, mtu);
 }
-EXPORT_SYMBOL(ipv4_mtu);
+EXPORT_INDIRECT_CALLABLE(ipv4_mtu);
 
 static void ip_del_fnhe(struct fib_nh_common *nhc, __be32 daddr)
 {

net/ipv6/route.c

@@ -2644,7 +2644,7 @@ INDIRECT_CALLABLE_SCOPE struct dst_entry *ip6_dst_check(struct dst_entry *dst,
 
 	return dst_ret;
 }
-EXPORT_SYMBOL(ip6_dst_check);
+EXPORT_INDIRECT_CALLABLE(ip6_dst_check);
 
 static struct dst_entry *ip6_negative_advice(struct dst_entry *dst)
 {
@@ -3115,7 +3115,7 @@ INDIRECT_CALLABLE_SCOPE unsigned int ip6_mtu(const struct dst_entry *dst)
 
 	return mtu - lwtunnel_headroom(dst->lwtstate, mtu);
 }
-EXPORT_SYMBOL(ip6_mtu);
+EXPORT_INDIRECT_CALLABLE(ip6_mtu);
 
 /* MTU selection:
  * 1. mtu on route is locked - use it