kcsan, seqlock: Support seqcount_latch_t

While fuzzing an arm64 kernel, Alexander Potapenko reported:

| BUG: KCSAN: data-race in ktime_get_mono_fast_ns / timekeeping_update
|
| write to 0xffffffc082e74248 of 56 bytes by interrupt on cpu 0:
|  update_fast_timekeeper kernel/time/timekeeping.c:430 [inline]
|  timekeeping_update+0x1d8/0x2d8 kernel/time/timekeeping.c:768
|  timekeeping_advance+0x9e8/0xb78 kernel/time/timekeeping.c:2344
|  update_wall_time+0x18/0x38 kernel/time/timekeeping.c:2360
|  [...]
|
| read to 0xffffffc082e74258 of 8 bytes by task 5260 on cpu 1:
|  __ktime_get_fast_ns kernel/time/timekeeping.c:372 [inline]
|  ktime_get_mono_fast_ns+0x88/0x174 kernel/time/timekeeping.c:489
|  init_srcu_struct_fields+0x40c/0x530 kernel/rcu/srcutree.c:263
|  init_srcu_struct+0x14/0x20 kernel/rcu/srcutree.c:311
|  [...]
|
| value changed: 0x000002f875d33266 -> 0x000002f877416866
|
| Reported by Kernel Concurrency Sanitizer on:
| CPU: 1 UID: 0 PID: 5260 Comm: syz.2.7483 Not tainted 6.12.0-rc3-dirty #78

This is a false positive data race between a seqcount latch writer and a reader
accessing stale data. Since its introduction, KCSAN has never understood the
seqcount_latch interface (due to being unannotated).

Unlike the regular seqlock interface, the seqcount_latch interface for latch
writers never has had a well-defined critical section, making it difficult to
teach tooling where the critical section starts and ends.

Introduce an instrumentable (non-raw) seqcount_latch interface, with
which we can clearly denote writer critical sections. This both helps
readability and tooling like KCSAN to understand when the writer is done
updating all latch copies.

Fixes: 88ecd153 ("seqlock, kcsan: Add annotations for KCSAN")
Reported-by: Alexander Potapenko <glider@google.com>
Co-developed-by: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Signed-off-by: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20241104161910.780003-4-elver@google.com