md/raid0: fix NULL pointer dereference in create_strip_zones() for dm-raid

md/raid0: fix NULL pointer dereference in create_strip_zones() for dm-raid

Commit 2107457e31fa ("md/raid0: Move queue limit setup before r0conf
initialization") dereference mddev->gendisk unconditionally, which is
NULL for dm-raid.

Fix this problem by reverting to old codes for dm-raid.

Link: https://lore.kernel.org/linux-raid/20251116021816.107648-1-yukuai@fnnas.com
Fixes: 2107457e31fa ("md/raid0: Move queue limit setup before r0conf initialization")
Reported-and-tested-by: Changhui Zhong <czhong@redhat.com>
Closes: https://lore.kernel.org/all/CAGVVp+VqVnvGeneUoTbYvBv2cw6GwQRrR3B-iQ-_9rVfyumoKA@mail.gmail.com/
Signed-off-by: Yu Kuai <yukuai@fnnas.com>
Reviewed-by: Xiao Ni <xni@redhat.com>
Reviewed-by: Li Nan <linan122@huawei.com>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>