sgsn: gtp: Use new API to send PDP DEL CTX REQ without freeing ctx locally
Previous API freed the ctx immediatelly after sending the packet, which
triggered a call to cb_delete_context() and dropped the entire
sgsn_pdp_ctx before the PDP DEL CTX ACCEPT was received. This new API
won't free the pdp ctx and we can tear down everything once we receive
the ACCEPT in cb_conf.
cb_conf is not automatically freed at cb_conf, user needs to free it, so
we need to remove setting pctx->lib to NULL in cb_conf to avoid leaking the
pdp ctx, as it needs to be freed inside sgsn_pdp_ctx_free().
Depends: osmo-ggsn (libgtp) Change-Id I29d366253bb98dcba328c7ce8aa3e4daf8f75e6c.
Change-Id: I304c59de5d137b81de3c6df0fdbe911ae3dbd1f3