gtp: Implement GTP echo response

Adding GTP device through ip link creates the situation where
there is no userspace daemon which would handle GTP messages
(Echo Request for example). GTP-U instance which would not respond
to echo requests would violate GTP specification.

When GTP packet arrives with GTP_ECHO_REQ message type,
GTP_ECHO_RSP is send to the sender. GTP_ECHO_RSP message
should contain information element with GTPIE_RECOVERY tag and
restart counter value. For GTPv1 restart counter is not used
and should be equal to 0, for GTPv0 restart counter contains
information provided from userspace(IFLA_GTP_RESTART_COUNT).

Signed-off-by: Wojciech Drewek <wojciech.drewek@intel.com>
Suggested-by: Harald Welte <laforge@gnumonks.org>
Reviewed-by: Harald Welte <laforge@gnumonks.org>
Tested-by: Harald Welte <laforge@gnumonks.org>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>