ipa: use LSBs of file descriptor as SLS in IPA->M3UA direction

when IPA/SCCPlite traffic traverses the osmo_ss7 stack, so far the
SLS of the MTP3 label was always set to 0 (default initialization).

However, in order to achieve a better distribution of signaling traffic
in SS7 networks that actually utilize the SLS to select any links (or
ASPs), it makes sense to use non-zero values.

Instead of introducing some kind of new, configurable attribute for
each ASP (IPA client connection), let's simply use the lower 4 bits of
the file descriptor integer.  Those file descriptors should have a
roughly equal distribution, resulting in traffic (from multiple IPA
clients) to be spread across the 4-bit SLS number space.

Also, this mechanism ensures that traffic from one IPA client will
always get the same SLS and hence routed the same way in any underlying
CS7/SS7 network.

Change-Id: Ice7bab997b84cfed00c7d6d780c70f4e9fac6002
Related: SYS#6543
---
M src/ipa.c
M src/osmo_ss7.c
M src/xua_internal.h
3 files changed, 35 insertions(+), 6 deletions(-)