Fix bug in scheduler lock mechanism

nr_mac_prepare_ra_ue() is called with the scheduler being locked via
NR_SCHED_ENSURE_LOCKED(), but if there is no free RA process, it
unlocks the scheduler via NR_SCHED_UNLOCK().  This is commit fixes this.

In short, when calling nr_mac_prepare_ra_ue(), the scheduler is locked,
and should always return with the scheduler being locked.

openair2/LAYER2/NR_MAC_gNB/config.c

@@ -594,7 +594,6 @@ bool nr_mac_prepare_ra_ue(gNB_MAC_INST *nrmac, uint32_t rnti, NR_CellGroupConfig
   }
   if (ra_index == NR_NB_RA_PROC_MAX) {
     LOG_E(NR_MAC, "RA processes are not available for CFRA RNTI %04x\n", rnti);
-    NR_SCHED_UNLOCK(&nrmac->sched_lock);
     return false;
   }
   NR_RA_t *ra = &cc->ra[ra_index];