Changes

llc: Mark old/current tlli as all 1's when unassigning LLME

TS 44.064 section 8.3.3 (and other sections) talk about special
unassigned value of "all 1's", but I couldn't find any reference to a
"all 0's" specific value/meaning.
In practice in the code this may not be super important since those
values may not ve checked due to the FSM state, but in any case they are
initially set to all 1's, so it makes total sense to re-set them to the
same unassigned value instead of a randomly chosen all 0's value.

This is a port of osmo-sgsn.git 35c178e84d25a216a5c2d9bd1320d072e36937fa.

Change-Id: I9db198a31963db015e887e33baf3c32b46e2b11a