md: initialize 'writes_pending' while allocating mddev

Currently 'writes_pending' is initialized in pers->run for raid1/5/10,
and it's freed while deleing mddev, instead of pers->free. pers->run can
be called multiple times before mddev is deleted, and a helper
mddev_init_writes_pending() is used to prevent 'writes_pending' to be
initialized multiple times, this usage is safe but a litter weird.

On the other hand, 'writes_pending' is only initialized for raid1/5/10,
however, it's used in common layer, for example:

array_state_store
 set_in_sync
  if (!mddev->in_sync) -> in_sync is used for all levels
   // access writes_pending

There might be some implicit dependency that I don't recognized to make
sure 'writes_pending' can only be accessed for raid1/5/10, but there are
no comments about that.

By the way, it make sense to initialize 'writes_pending' in common layer
because there are already three levels use it.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230825030956.1527023-3-yukuai1@huaweicloud.com