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.

Change-Id: I18dc9d30417b0c5b2e579660d4a087d93445f956
AI-Assisted: yes (Claude)
---
M src/common/lchan.c
1 file changed, 6 insertions(+), 0 deletions(-)