sgsn: Fix crash using new libgtp cb_recovery2 API

When PDP CTX CREATE ACK is received with an increased RestartCtr, cb_recovery2
is called first, which will dettach ggsn from al pdp ctx (free the
pdp_t). But when giving control back from the ctrl, libgtp still uses
that freed ctx and sends it back to osmo-sgsn through cb_conf().

As specs state in any case that we need to handle the message containing
the increased RestartCtr as valid, we then need to avoid freeing the pdp
ctx and leave handling for later in cb_conf.

Depends: osmo-ggsn (libgtp) Change-Id I53e92298f2f6b84d662a3300d922e8c2ccb178bc.
Change-Id: I0989c00e18ca95a099e1a312940eaac71957b444
diff --git a/include/osmocom/sgsn/gprs_sgsn.h b/include/osmocom/sgsn/gprs_sgsn.h
index 8eba2d4..6f16dc7 100644
--- a/include/osmocom/sgsn/gprs_sgsn.h
+++ b/include/osmocom/sgsn/gprs_sgsn.h
@@ -362,7 +362,7 @@
 struct sgsn_ggsn_ctx *sgsn_ggsn_ctx_by_addr(struct in_addr *addr);
 struct sgsn_ggsn_ctx *sgsn_ggsn_ctx_find_alloc(uint32_t id);
 void sgsn_ggsn_ctx_drop_pdp(struct sgsn_pdp_ctx *pctx);
-int sgsn_ggsn_ctx_drop_all_pdp(struct sgsn_ggsn_ctx *ggsn);
+int sgsn_ggsn_ctx_drop_all_pdp_except(struct sgsn_ggsn_ctx *ggsn, struct sgsn_pdp_ctx *except);
 void sgsn_ggsn_ctx_add_pdp(struct sgsn_ggsn_ctx *ggc, struct sgsn_pdp_ctx *pdp);
 void sgsn_ggsn_ctx_remove_pdp(struct sgsn_ggsn_ctx *ggc, struct sgsn_pdp_ctx *pdp);