gtp: consolidate gtp socket rx path

Add network device to gtp context in preparation for splitting
the TEID from the network device.

Use this to rework the socker rx path. Move the common RX part
of v0 and v1 into a helper. Also move the final rx part into
that helper as well.

Signed-off-by: Andreas Schultz <aschultz@tpip.net>
Signed-off-by: David S. Miller <davem@davemloft.net>