MDEV-6934: os_event_wait_time_low(): wait time calculation is messed up

Merged Facebook commit bdab302a7e3c37da21a1bffe1550cdbe6c906695
by Inaam Rana from https://github.com/facebook/mysql-5.6.

In os_event_wait_time_low() the logic to calculate abs_time
for wait is broken. The bug has been present at least since
5.5. It gets acutely sensitized when sub-second wait intervals
are passed. It is particularly relevant to us because the
page_cleaner thread will mostly request sub-second wait
intervals. This can potentially lead to a near tight loop
behaviour of page_cleaner with much less sleep then what we'd
actually expect.

storage/innobase/os/os0sync.cc

@@ -686,7 +686,7 @@ os_event_wait_time_low(
 		tv.tv_usec += time_in_usec;
 
 		if ((ulint) tv.tv_usec >= MICROSECS_IN_A_SECOND) {
-			tv.tv_sec += time_in_usec / MICROSECS_IN_A_SECOND;
+			tv.tv_sec += tv.tv_usec / MICROSECS_IN_A_SECOND;
 			tv.tv_usec %= MICROSECS_IN_A_SECOND;
 		}
 

storage/xtradb/os/os0sync.cc

@@ -686,7 +686,7 @@ os_event_wait_time_low(
 		tv.tv_usec += time_in_usec;
 
 		if ((ulint) tv.tv_usec >= MICROSECS_IN_A_SECOND) {
-			tv.tv_sec += time_in_usec / MICROSECS_IN_A_SECOND;
+			tv.tv_sec += tv.tv_usec / MICROSECS_IN_A_SECOND;
 			tv.tv_usec %= MICROSECS_IN_A_SECOND;
 		}