tbf: Add BTS::ms_alloc method

Currently the code that creates the MS objects with tbf.cpp is
duplicated.

This commit moves the corresponding code into a new method. Since
there is no TLLI available there, the GprsMsStorage::create_ms method
has been refactored into two variants, one with TLLI/direction and
one without.

Sponsored-by: On-Waves ehf
5 files changed