net: use indirect calls helpers for sk_exit_memory_pressure()

Florian reported a regression and sent a patch with the following
changelog:

<quote>
 There is a noticeable tcp performance regression (loopback or cross-netns),
 seen with iperf3 -Z (sendfile mode) when generic retpolines are needed.

 With SK_RECLAIM_THRESHOLD checks gone number of calls to enter/leave
 memory pressure happen much more often. For TCP indirect calls are
 used.

 We can't remove the if-set-return short-circuit check in
 tcp_enter_memory_pressure because there are callers other than
 sk_enter_memory_pressure.  Doing a check in the sk wrapper too
 reduces the indirect calls enough to recover some performance.

 Before,
 0.00-60.00  sec   322 GBytes  46.1 Gbits/sec                  receiver

 After:
 0.00-60.04  sec   359 GBytes  51.4 Gbits/sec                  receiver

 "iperf3 -c $peer -t 60 -Z -f g", connected via veth in another netns.
</quote>

It seems we forgot to upstream this indirect call mitigation we
had for years, lets do this instead.

[edumazet] - It seems we forgot to upstream this indirect call
             mitigation we had for years, let's do this instead.
           - Changed to INDIRECT_CALL_INET_1() to avoid bots reports.

Fixes: 4890b68 ("net: keep sk->sk_forward_alloc as small as possible")
Reported-by: Florian Westphal <[email protected]>
Link: https://lore.kernel.org/netdev/[email protected]/T/
Signed-off-by: Brian Vazquez <[email protected]>
Signed-off-by: Eric Dumazet <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>

Author: Brian Vazquez

net/core/sock.c

@@ -2818,7 +2818,8 @@ static void sk_enter_memory_pressure(struct sock *sk)
 static void sk_leave_memory_pressure(struct sock *sk)
 {
 	if (sk->sk_prot->leave_memory_pressure) {
-		sk->sk_prot->leave_memory_pressure(sk);
+		INDIRECT_CALL_INET_1(sk->sk_prot->leave_memory_pressure,
+				     tcp_leave_memory_pressure, sk);
 	} else {
 		unsigned long *memory_pressure = sk->sk_prot->memory_pressure;