md: add a new recovery_flag MD_RECOVERY_LAZY_RECOVER

This flag is used by llbitmap in later patches to skip raid456 initial
recover and delay building initial xor data to first write.

https://lore.kernel.org/linux-raid/20250829080426.1441678-10-yukuai1@huaweicloud.com


Signed-off-by: Yu Kuai <yukuai3@huawei.com>