Changes

ggsn: kernel gtpu: Support updating pdp ctx remote IP address and TEID

Whenever the SGSN sends the GGSN a UpdatePDPCtxReq, it may offer a new
remote IP address or/and remote TEID, eg. because it wants to establish
a Direct Tunnel and point the GTPU towards the RNC, or to point it back
to itself.
If the gtpu kernel is used, osmo-ggsn lacked updating the kernel with
the new remote data.
The gtp kernel module doesn't provide an efficient/explicit way to
update a pdp context keeping the4 same local IP+TEID and changing only
the remote remote IP+TEID, hence first destroy the pdp ctx in the gtp
kernel module and then recreate it.

This fixes test GGSN_Tests_v4_only.TC_pdp4_act_update_teid in
ttcn3-ggsn-test-kernel-net-next.

Related: OS#6523
Change-Id: I1fc48be5c0f177ccf6fbe97c003b4df44809c0fe