mctp: return -ENOPROTOOPT for unknown getsockopt options

In mctp_getsockopt(), unrecognized options currently return -EINVAL.
In contrast, mctp_setsockopt() returns -ENOPROTOOPT for unknown
options.

Update mctp_getsockopt() to also return -ENOPROTOOPT for unknown
options. This aligns the behavior of getsockopt() and setsockopt(),
and matches the standard kernel socket API convention for handling
unsupported options.

Fixes: 99ce45d5e7db ("mctp: Implement extended addressing")
Signed-off-by: Alok Tiwari <alok.a.tiwari@oracle.com>
Link: https://patch.msgid.link/20250902102059.1370008-1-alok.a.tiwari@oracle.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
diff --git a/net/mctp/af_mctp.c b/net/mctp/af_mctp.c
index df4e8cf..6855248 100644
--- a/net/mctp/af_mctp.c
+++ b/net/mctp/af_mctp.c
@@ -425,7 +425,7 @@ static int mctp_getsockopt(struct socket *sock, int level, int optname,
 		return 0;
 	}
 
-	return -EINVAL;
+	return -ENOPROTOOPT;
 }
 
 /* helpers for reading/writing the tag ioc, handling compatibility across the