net: fib_rules: consolidate IPv4 and DECnet ->default_pref() functions.

Both functions are equivalent, consolidate them since a following patch
needs a third implementation for multicast routing.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/fib_rules.h b/include/net/fib_rules.h
index c49086d..52bd9e6 100644
--- a/include/net/fib_rules.h
+++ b/include/net/fib_rules.h
@@ -114,4 +114,5 @@
 extern int			fib_default_rule_add(struct fib_rules_ops *,
 						     u32 pref, u32 table,
 						     u32 flags);
+extern u32			fib_default_rule_pref(struct fib_rules_ops *ops);
 #endif