common: stop buffering UL measurements when SACCH is deactivated
When the BSC sends RSL DEACT SACCH, the per-SACCH UL measurement drain stops (it runs on SACCH timing), but the producer in lchan_new_ul_meas() keeps appending the measurement contributions from every received TCH/SDCCH burst. After one SACCH period (104 frames) the 104-slot uplink measurement buffer fills up, yielding a flood of:
NOTICE measurement.c:336 no space for uplink measurement, num_ul_meas=104
Add a bool sacch_active flag to gsm_lchan, set to true in the common l1sap_chan_act() and clear in l1sap_chan_deact_sacch(). Guard lchan_new_ul_meas() with this flag so that measurements are silently discarded while SACCH is inactive - there is nothing to drain the buffer and no SACCH channel on which to report the results to the BSC.
common: reset lchan meas state in gsm_lchan_release()
lchan->meas (including interf_meas_num and num_ul_meas) is normally only reset by lchan_meas_reset() called from rsl_tx_chan_act_ack() on RSL CHANNEL ACTIVATION. Idle logical channels are never RSL-activated, so their measurement state is never reset via that path.
On an OML link re-establishment, osmo-bts does not exit: abis.c tears down the signalling links, the bts_shutdown FSM powers down all TRXs, and then waits for reconnect. The gsm_bts/trx/ts/lchan structures remain in memory, so stale interf_meas_num survives the reconnect.
This is why we're seeing these ERRORs while running ttcn3-bts-test:
(bts=0,trx=2,ts=4,ss=6) Not enough room to store interference report (0dBm)
Add a lchan_meas_reset() call to gsm_lchan_release(), which is called from gsm_ts_release() when the nm_channel_fsm enters state NM_CHAN_ST_OP_DISABLED_NOTINSTALLED. This is exactly the right moment: the radio is fully stopped, so no new samples arrive.
