[PATCHv2] SO_ZEROCOPY should return -EOPNOTSUPP rather than -ENOTSUPP

[Samuel Thibault <samuel.thibault@labri.fr>]

ENOTSUPP is documented as "should never be seen by user programs",
and thus not exposed in <errno.h>, and thus applications cannot safely
check against it (they get "Unknown error 524" as strerror). We should
rather return the well-known -EOPNOTSUPP.

This is similar to 2230a7ef5198 ("drop_monitor: Use correct error
code") and 4a5cdc604b9c ("net/tls: Fix return values to avoid
ENOTSUPP"), which did not seem to cause problems.

Signed-off-by: Samuel Thibault <samuel.thibault@labri.fr>

---
Difference with v1: use -EOPNOTSUPP instead of -ENOPROTOOPT.

diff --git a/net/core/sock.c b/net/core/sock.c
index 4ff806d71921..839eb076afee 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -1377,9 +1377,9 @@ int sock_setsockopt(struct socket *sock, int level, int optname,
 			if (!(sk_is_tcp(sk) ||
 			      (sk->sk_type == SOCK_DGRAM &&
 			       sk->sk_protocol == IPPROTO_UDP)))
-				ret = -ENOTSUPP;
+				ret = -EOPNOTSUPP;
 		} else if (sk->sk_family != PF_RDS) {
-			ret = -ENOTSUPP;
+			ret = -EOPNOTSUPP;
 		}
 		if (!ret) {
 			if (val < 0 || val > 1)