gtp: add notification mechanism

Like all other network functions, let's notify gtp context on creation and
deletion.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Tested-by: Gabriel Ganne <gabriel.ganne@6wind.com>
Acked-by: Harald Welte <laforge@gnumonks.org>
Signed-off-by: David S. Miller <davem@davemloft.net>