bpf, net: Fix build issue when net ns not configured

Fix a redefinition of 'net_gen_cookie' error that was overlooked
when net ns is not configured.

Fixes: f318903c ("bpf: Add netns cookie and enable it for bpf cgroup hooks")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

include/net/net_namespace.h

@@ -228,6 +228,8 @@ extern struct list_head net_namespace_list;
 struct net *get_net_ns_by_pid(pid_t pid);
 struct net *get_net_ns_by_fd(int fd);
 
+u64 net_gen_cookie(struct net *net);
+
 #ifdef CONFIG_SYSCTL
 void ipx_register_sysctl(void);
 void ipx_unregister_sysctl(void);
@@ -276,8 +278,6 @@ static inline int check_net(const struct net *net)
 
 void net_drop_ns(void *);
 
-u64 net_gen_cookie(struct net *net);
-
 #else
 
 static inline struct net *get_net(struct net *net)
@@ -305,11 +305,6 @@ static inline int check_net(const struct net *net)
 	return 1;
 }
 
-static inline u64 net_gen_cookie(struct net *net)
-{
-	return 0;
-}
-
 #define net_drop_ns NULL
 #endif