f2fs: annotate lockless last_time[] accesses

f2fs stores mount-wide activity timestamps in sbi->last_time[] and
samples them from background discard, GC, and balance paths without a
dedicated lock. The timestamps are used as best-effort heuristics to
decide whether background work should run now or sleep a bit longer.

The current helpers use plain loads and stores, so KCSAN can report races
between frequent foreground updates and background readers. Exact
freshness is not required here, but the intentional lockless accesses
should be marked explicitly.

Use WRITE_ONCE() in f2fs_update_time() and READ_ONCE() in
f2fs_time_over() and f2fs_time_to_wait(). This preserves the existing
heuristic behavior and avoids adding locking to hot paths.

Signed-off-by: Cen Zhang <zzzccc427@gmail.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

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