netns: fix get_net_ns_by_fd(int pid) typo

The argument to get_net_ns_by_fd() is a /proc/$PID/ns/net file
descriptor not a pid.  Fix the typo.
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Rami Rosen <roszenrami@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/net_namespace.h

@@ -170,7 +170,7 @@ static inline struct net *copy_net_ns(unsigned long flags,
 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 pid);
+struct net *get_net_ns_by_fd(int fd);
 
 #ifdef CONFIG_SYSCTL
 void ipx_register_sysctl(void);