Fix Lb/A SCCP conn lookup after recent regression in optimization patch

In osmo-bsc, there's currently 0..1 Lb links and 0..N A links, where N
is the number of MSC, but links can be shared in the underlaying stack
(struct osmo_sccp_instance), hence range 0..N of different
osmo_sccp_instance (identified by PC).
Even more, the Lb and A link can share the same underlaying stack, so
osmo-bsc can end up with only 1 struct osmo_sccp_instance shared by all
the above mentioned links in case all are configured under the same PC.
Total range A+Lb is 0..(1+N).

A struct gsm_subscriber_conn stores 2 struct sccp_instance*, one for
Lb (conn->lcs.lb.*)and one for A (conn->sccp.*).
They can actually point to the same sccp_instance or to different ones,
as explained above, depending on the configured setup. In any case, a
gsm_subscriber_conn needs 2 rb_nodes since it can hold
any of the 2 conn_ids independently (A or Lb).

The previous patch forgot to add that 2nd rb_node as well as some
initialization and release code for the Lb conn. This patch addresses
that.

When the 2nd rb_node, a problem when iterating the rbtree appears: how to
find out the "conn" pointer from the rb_node pointer, since the rb_node pointer
can be any of the 2 rb_nodes inside the struct at a different offsets.

In order to solve that problem, a new struct bscp_sccp_conn_node is
added, which holds all the relevant information used by the rbtree lookup code
in a generic way (rb_node and conn_id), plus a backpointer to the struct
bsc_gsm_subcriber it relates too.

Fixes: 85062ccad31e9fb73e0256a5ee556c6ae0a16449
Change-Id: If42d93adee71d646766929a09bc01ae92b734ef3
---
M include/osmocom/bsc/gsm_data.h
M src/osmo-bsc/bsc_sccp.c
M src/osmo-bsc/bsc_subscr_conn_fsm.c
M src/osmo-bsc/bsc_vty.c
M src/osmo-bsc/handover_fsm.c
M src/osmo-bsc/lb.c
M src/osmo-bsc/lchan_rtp_fsm.c
M src/osmo-bsc/osmo_bsc_sigtran.c
M tests/bsc/bsc_test.c
9 files changed, 111 insertions(+), 54 deletions(-)