xsk: do not call synchronize_net() under RCU read lock

The XSKMAP update and delete functions called synchronize_net(), which
can sleep. It is not allowed to sleep during an RCU read section.

Instead we need to make sure that the sock sk_destruct (xsk_destruct)
function is asynchronously called after an RCU grace period. Setting
the SOCK_RCU_FREE flag for XDP sockets takes care of this.

Fixes: fbfc504a24 ("bpf: introduce new bpf AF_XDP map type BPF_MAP_TYPE_XSKMAP")
Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

net/xdp/xsk.c

@@ -744,6 +744,8 @@ static int xsk_create(struct net *net, struct socket *sock, int protocol,
 	sk->sk_destruct = xsk_destruct;
 	sk_refcnt_debug_inc(sk);
 
+	sock_set_flag(sk, SOCK_RCU_FREE);
+
 	xs = xdp_sk(sk);
 	mutex_init(&xs->mutex);
 	spin_lock_init(&xs->tx_completion_lock);