CLEANUP: memory: make the fault injection code use the OTHER_LOCK label

The mem_should_fail() function sets a lock while it's building its
messages, and when this was done there was no relevant label available
hence the confusing use of START_LOCK. Now OTHER_LOCK is available for
such use cases, so let's switch to this one instead as START_LOCK is
going to disappear.
diff --git a/src/memory.c b/src/memory.c
index 140671b..6555913 100644
--- a/src/memory.c
+++ b/src/memory.c
@@ -594,7 +594,7 @@
 		else
 			ret = 0;
 	}
-	HA_SPIN_LOCK(START_LOCK, &mem_fail_lock);
+	HA_SPIN_LOCK(OTHER_LOCK, &mem_fail_lock);
 	n = snprintf(&mem_fail_str[mem_fail_cur_idx * MEM_FAIL_MAX_CHAR],
 	    MEM_FAIL_MAX_CHAR - 2,
 	    "%d %.18s %d %d", mem_fail_cur_idx, pool->name, ret, tid);
@@ -607,7 +607,7 @@
 	mem_fail_cur_idx++;
 	if (mem_fail_cur_idx == MEM_FAIL_MAX_STR)
 		mem_fail_cur_idx = 0;
-	HA_SPIN_UNLOCK(START_LOCK, &mem_fail_lock);
+	HA_SPIN_UNLOCK(OTHER_LOCK, &mem_fail_lock);
 	return ret;
 
 }