gtp: Allow to create GTP device without FDs

Currently, when the user wants to create GTP device, he has to
provide file handles to the sockets created in userspace (IFLA_GTP_FD0,
IFLA_GTP_FD1). This behaviour is not ideal, considering the option of
adding support for GTP device creation through ip link. Ip link
application is not a good place to create such sockets.

This patch allows to create GTP device without providing
IFLA_GTP_FD0 and IFLA_GTP_FD1 arguments. If the user sets
IFLA_GTP_CREATE_SOCKETS attribute, then GTP module takes care
of creating UDP sockets by itself. Sockets are created with the
commonly known UDP ports used for GTP protocol (GTP0_PORT and
GTP1U_PORT). In this case we don't have to provide encap_destroy
because no extra deinitialization is needed, everything is covered
by udp_tunnel_sock_release.

Note: GTP instance created with only this change applied, does
not handle GTP Echo Requests. This is implemented in the following
patch.

Signed-off-by: Wojciech Drewek <wojciech.drewek@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

include/uapi/linux/if_link.h

@@ -887,6 +887,7 @@ enum {
 	IFLA_GTP_FD1,
 	IFLA_GTP_PDP_HASHSIZE,
 	IFLA_GTP_ROLE,
+	IFLA_GTP_CREATE_SOCKETS,
 	__IFLA_GTP_MAX,
 };
 #define IFLA_GTP_MAX (__IFLA_GTP_MAX - 1)