soreuseport: fix NULL ptr dereference SO_REUSEPORT after bind

Marc Dionne discovered a NULL pointer dereference when setting
SO_REUSEPORT on a socket after it is bound.
This patch removes the assumption that at least one socket in the
reuseport group is bound with the SO_REUSEPORT option before other
bind calls occur.

Fixes: e32ea7e74727 ("soreuseport: fast reuseport UDP socket selection")
Reported-by: Marc Dionne <marc.c.dionne@gmail.com>
Signed-off-by: Craig Gallek <kraig@google.com>
Tested-by: Marc Dionne <marc.dionne@auristor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/sock_reuseport.h b/include/net/sock_reuseport.h
index 7dda3d7..aecd303 100644
--- a/include/net/sock_reuseport.h
+++ b/include/net/sock_reuseport.h
@@ -16,7 +16,7 @@
 };
 
 extern int reuseport_alloc(struct sock *sk);
-extern int reuseport_add_sock(struct sock *sk, const struct sock *sk2);
+extern int reuseport_add_sock(struct sock *sk, struct sock *sk2);
 extern void reuseport_detach_sock(struct sock *sk);
 extern struct sock *reuseport_select_sock(struct sock *sk,
 					  u32 hash,