ntp: Clamp PLL update interval

Clamp update interval to reduce PLL gain with low sampling rate (e.g.
intermittent network connection) to avoid instability.

The clamp roughly corresponds to the loop time constant, it's 8 * poll
interval for SHIFT_PLL 2 and 32 * poll interval for SHIFT_PLL 4. This
gives good results without affecting the gain in normal conditions where
ntpd skips only up to seven consecutive samples.

Signed-off-by: Miroslav Lichvar <[email protected]>
Acked-by: john stultz <[email protected]>
LKML-Reference: <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>

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