dm-raid: remove useless checking in raid_message()

mainline inclusion
from mainline-v6.5-rc1
commit 955a257d
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8OPEK
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=955a257d69e44cea09b0375b8f2f3d4d9fcf7b4e



--------------------------------

md_wakeup_thread() handle the case that pass in md_thread is NULL, there
is no need to check this.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230523021017.3048783-3-yukuai1@huaweicloud.com


Signed-off-by: Li Lingfeng <lilingfeng3@huawei.com>