xsk, net: Make sock_def_readable() have external linkage

XDP sockets use the default implementation of struct sock's
sk_data_ready callback, which is sock_def_readable(). This function
is called in the XDP socket fast-path, and involves a retpoline. By
letting sock_def_readable() have external linkage, and being called
directly, the retpoline can be avoided.
Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200120092917.13949-1-bjorn.topel@gmail.com

include/net/sock.h

@@ -2612,4 +2612,6 @@ static inline bool sk_dev_equal_l3scope(struct sock *sk, int dif)
 	return false;
 }
 
+void sock_def_readable(struct sock *sk);
+
 #endif	/* _SOCK_H */

net/core/sock.c

@@ -2786,7 +2786,7 @@ static void sock_def_error_report(struct sock *sk)
 	rcu_read_unlock();
 }
 
-static void sock_def_readable(struct sock *sk)
+void sock_def_readable(struct sock *sk)
 {
 	struct socket_wq *wq;
 

net/xdp/xsk.c

@@ -217,7 +217,7 @@ static int xsk_rcv(struct xdp_sock *xs, struct xdp_buff *xdp)
 static void xsk_flush(struct xdp_sock *xs)
 {
 	xskq_prod_submit(xs->rx);
-	xs->sk.sk_data_ready(&xs->sk);
+	sock_def_readable(&xs->sk);
 }
 
 int xsk_generic_rcv(struct xdp_sock *xs, struct xdp_buff *xdp)