timekeeping: Settle competing time_offset and time_adjust skew

time_offset (the exponential PLL phase slew) and time_adjust (the
linear adjtime() slew) can be asked to move the clock in opposite
directions. second_overflow() folds only their *net* into the per-tick
skew_delta, so the cancelling overlap would never be drained from
either tracker by the per-tick code — and if they cancel exactly,
skew_delta is zero and neither converges at all.

Arguably we could just let one of them entirely cancel out the other
immediately, but that would be a change in userspace-visible behaviour.

Instead, preserve the existing behaviour by calculating the "conflict"
portion between the opposing skew each second, and transferring that
amount directly from one tracker to the other.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Assisted-by: Kiro:claude-opus-4.8
Link: https://patch.msgid.link/20260621220051.1030462-7-dwmw2@infradead.org

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