gtp: Implement GTP echo request

Adding GTP device through ip link creates the situation where
GTP instance is not able to send GTP echo requests.
Echo requests are used to check if GTP peer is still alive.
With this patch, gtp_genl_ops are extended by new cmd (GTP_CMD_ECHOREQ)
which allows to send echo request in the given version of GTP
protocol (v0 or v1), from the given ms address to he given
peer. TID is not inclued because in all path management
messages it should be equal to 0.

When GTP echo response is detected, multicast message is
send to everyone in the gtp_genl_family. Message contains
GTP version, ms address and peer address.
Suggested-by: Harald Welte <laforge@gnumonks.org>
Signed-off-by: Wojciech Drewek <wojciech.drewek@intel.com>
Reviewed-by: Harald Welte <laforge@gnumonks.org>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

include/uapi/linux/gtp.h

@@ -8,6 +8,7 @@ enum gtp_genl_cmds {
 	GTP_CMD_NEWPDP,
 	GTP_CMD_DELPDP,
 	GTP_CMD_GETPDP,
+	GTP_CMD_ECHOREQ,
 
 	GTP_CMD_MAX,
 };