timekeeping: Drive time_adjust skew via per-tick ntp_error transfer

The legacy adjtime() slew (ADJ_OFFSET_SINGLESHOT) was the last user of
tick_length != tick_length_base: it slewed the clock by inflating
tick_length directly, which delivered the correction imprecisely (e.g.
delivering only 4997.5µs when asked for a 5ms skew).

Deliver it accurately through the same per-tick mechanism that is now
used for time_offset, allowing it to contribute to skew_delta and thus
drive the delivery through ntp_error and mult selection.

To allow for accurate accounting, store the sub-microsecond part of
time_adjust is separately, while keeping time_adjust in microseconds
as that's the external API.

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/20260621220051.1030462-6-dwmw2@infradead.org

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