timekeeping: Use READ_ONCE/WRITE_ONCE() for ktime_sec to prevent tearing

The timekeeper update path uses a bulk memcpy() to synchronize the
timekeeper structure, which is not guaranteed to be atomic. This allows for
torn reads in ktime_get_seconds() which bypasses the sequence counter
protection for performance.

To prevent reading a torn ktime_sec value, enforce atomic-like
access by using WRITE_ONCE() for the critical field before the bulk
memcpy() in timekeeping_update_from_shadow(). Correspondingly, use
READ_ONCE() in ktime_get_seconds() to ensure a fresh, consistent load
from memory.

The same was done for xtime_sec and ktime_get_real_seconds() in commit
d7fc133bf91f ("timekeeping: Use READ_ONCE/WRITE_ONCE() for xtime_sec to
prevent tearing").

Signed-off-by: Thomas Weißschuh (Schneider Electric) <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Acked-by: John Stultz <jstultz@google.com>
Link: https://patch.msgid.link/20260903-timekeeping-ktime_sec-v1-1-b33017536aa9@linutronix.de

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