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.

Change-Id: I3943c788cab5d2411b06ac681d4d412852bac0a7
AI-Assisted: yes (Claude)
---
M include/osmo-bts/lchan.h
M src/common/l1sap.c
M src/common/measurement.c
M tests/meas/meas_test.c
4 files changed, 11 insertions(+), 0 deletions(-)