lapdm: Make sure that the msgb_l3len(msg) == length...

This code should not play with the internals of the msgb like this,
this code got introduced in af48bed55607931307 and is breaking the
osmo-bts usecase of forwarding an RSL message.

Add a test case that fails without the new code. I would prefer if
we could get rid of the manipulating the msgb like this, it is prone
to errors like this one.
3 files changed