timekeeping: Don't use seqcount loop in ktime_mono_to_any() on 64-bit systems

ktime_mono_to_any() only fetches the offset inside the loop. This is a
single word on 64-bit CPUs, and seqcount_read_begin() implies a full SMP
barrier.

Use READ_ONCE() to fetch the offset instead of doing a seqcount loop on
64-bit and add the matching WRITE_ONCE()'s to update the offsets in
tk_set_wall_to_mono() and tk_update_sleep_time().

[ tglx: Get rid of the #ifdeffery ]

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20240910-mgtime-v3-1-84406ed53fad@kernel.org