md: clean up invalid BUG_ON in md_ioctl

[ Upstream commit 9dd8702e7cd28ebf076ff838933f29cf671165ec ]

'disk->private_data' is set to mddev in md_alloc() and never set to NULL,
and users need to open mddev before submitting ioctl. So mddev must not
have been freed during ioctl, and there is no need to check mddev here.
Clean up it.

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/20240226031444.3606764-4-linan666@huaweicloud.com


Signed-off-by: Sasha Levin <sashal@kernel.org>
(cherry picked from commit 5c11581df1f58c43ce8b2e9c14184ab1f75c883f)
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>