gtp: uapi: fix GTPA_MAX

Subtract one to __GTPA_MAX, otherwise GTPA_MAX is off by 2.

Fixes: 459aa660 ("gtp: add initial driver for datapath of GPRS Tunneling Protocol (GTP-U)")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

include/uapi/linux/gtp.h

@@ -33,6 +33,6 @@ enum gtp_attrs {
 	GTPA_PAD,
 	__GTPA_MAX,
 };
-#define GTPA_MAX (__GTPA_MAX + 1)
+#define GTPA_MAX (__GTPA_MAX - 1)
 
 #endif /* _UAPI_LINUX_GTP_H_ */