MINOR: clock: use ltid_bit in clock_report_idle()

Since commit cc7a11ee3 ("MINOR: threads: set the tid, ltid and their bit
in thread_cfg") we ought not use (1UL << thr) to get the group mask for
thread <thr>, but (ha_thread_info[thr].ltid_bit). clock_report_idle()
needs this.

This also implies not using all_threads_mask anymore but taking the mask
from the tgroup since it becomes relative now.
diff --git a/src/clock.c b/src/clock.c
index 3b9e9ff..099057f 100644
--- a/src/clock.c
+++ b/src/clock.c
@@ -276,7 +276,8 @@
 	uint thr;
 
 	for (thr = 0; thr < MAX_THREADS; thr++) {
-		if (!(all_threads_mask & (1UL << thr)))
+		if (!ha_thread_info[thr].tg ||
+		    !(ha_thread_info[thr].tg->threads_enabled & ha_thread_info[thr].ltid_bit))
 			continue;
 		total += HA_ATOMIC_LOAD(&ha_thread_ctx[thr].idle_pct);
 		rthr++;