LLC: Store the XID inside the LLC Entity, not LLC Mgmg Entity
For every logical session between a MS and the SGSN, there is one LLME
(LLC Management Entity) and a set of LLEs (Logical Link Entities): One
for each SAPI.
The XID procedure used to establish LLC configuration values such as
N201 (MTU) parameters happens on each LLE separately. The negotiated
parameters only affect that one LLE (SAPI) and are not global.
Still, the OsmoSGSN LLC code has the "struct llist_head *xid" member as
part of the gprs_llc_llme, and not as part of the gprs_llc_lle. This
list is a cache of the XID fields we have sent with the last XID
request, which is used in processing the response from the MS.
If two XID handshakes were to occur concurrently on two LLEs, the state
between them would get messed up. It must be maintained separately for
each LLE.
Closes: OS#3955
Change-Id: Iaeb54ca5ac58391be45e56c2e721f531969f3a9e