When we switched away from the optimized C version
things stopped being monotonic.

The problem is that if we run this with interrupts disabled, we can
see the interrupt pending because the counter reached the limit value.
When this happens the counter has bit 31 set, and the low bits start
counting again from zero.

Reported by Martin Habets.
Signed-off-by: David S. Miller <davem@davemloft.net>