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kernel: fix todfix() race

we have to recheck the condition under tod lock, otherwise
another process can come in and updated tod.last and
tod.off and once we have the lock, we would make time
jump backwards.

--- a/sys/src/9/port/tod.c

+++ b/sys/src/9/port/tod.c

@@ -198,11 +198,13 @@

 	uvlong x;

 	ticks = fastticks(nil);

+	diff = ticks - tod.last;

+	if(diff <= tod.hz)

+		return;

+	ilock(&tod);

 	diff = ticks - tod.last;

 	if(diff > tod.hz){

-		ilock(&tod);

-

 		/* convert to epoch */

 		mul64fract(&x, diff, tod.multiplier);

 if(x > 30000000000ULL) iprint("todfix %llud\n", x);

@@ -211,9 +213,8 @@

 		/* protect against overflows */

 		tod.last = ticks;

 		tod.off = x;

-

-		iunlock(&tod);

 	}

+	iunlock(&tod);

 }

 long