gtp: merge gtp_get_net and gtp_genl_find_dev

Both function are always used together with the final goal to
get the gtp_dev. This simplifies the code by merging them together.

The netdevice lookup is changed to use the regular dev_get_by_index.
The gtp netdevice list is now only used to find the PDP contexts for
imcomming packets. It can be completely eliminated Once the TEID
hash is moved into the GTP socket.

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