md: merge the check of capabilities into md_ioctl_valid()

There is no functional change. Just to make code cleaner.

Signed-off-by: Li Nan <linan122@huawei.com>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20240226031444.3606764-2-linan666@huaweicloud.com