USB: serial: garmin_gps: use struct_size() to allocate pkt

Use the struct_size macro to calculate the size of the pkt, which
includes a trailing flexible array.
Suggested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/garmin_gps.c

@@ -267,8 +267,7 @@ static int pkt_add(struct garmin_data *garmin_data_p,
 
 	/* process only packets containing data ... */
 	if (data_length) {
-		pkt = kmalloc(sizeof(struct garmin_packet)+data_length,
-								GFP_ATOMIC);
+		pkt = kmalloc(struct_size(pkt, data, data_length), GFP_ATOMIC);
 		if (!pkt)
 			return 0;