net: Dereference pointer-value of sk_prot->memory_pressure

2e685cad57 (tcp_memcontrol: Kill struct tcp_memcontrol) falsly modified
the access to memory_pressure of sk->sk_prot->memory_pressure. The patch
did modify the memory_pressure-field of struct cg_proto, but not the one
of struct proto.

So, the access to sk_prot->memory_pressure should not be changed.

Acked-by: Eric Dumazet <edumazet@google.com>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -1137,7 +1137,7 @@ static inline bool sk_under_memory_pressure(const struct sock *sk)
 	if (mem_cgroup_sockets_enabled && sk->sk_cgrp)
 		return !!sk->sk_cgrp->memory_pressure;
 
-	return !!sk->sk_prot->memory_pressure;
+	return !!*sk->sk_prot->memory_pressure;
 }
 
 static inline void sk_leave_memory_pressure(struct sock *sk)