md/raid1: change r1conf->r1bio_pool to a pointer type

md/raid1: change r1conf->r1bio_pool to a pointer type

In raid1_reshape(), newpool is a stack variable.
mempool_init() initializes newpool->wait with the stack address.
After assigning newpool to conf->r1bio_pool, the wait queue
need to be reinitialized, which is not ideal.

Change raid1_conf->r1bio_pool to a pointer type and
replace mempool_init() with mempool_create_kmalloc_pool() to
avoid referencing a stack-based wait queue.

Signed-off-by: Wang Jinchao <wangjinchao600@gmail.com>
Link: https://lore.kernel.org/linux-raid/20250707012711.376844-2-yukuai1@huaweicloud.com
Signed-off-by: Yu Kuai <yukuai3@huawei.com>