blk-mq: fix an IS_ERR() vs NULL bug

mainline inclusion
from mainline-v5.14-rc1
commit 52d7e288
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I81XCK

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=52d7e288444906aa5c99888e80a9cc1a1423ed92



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The __blk_mq_alloc_disk() function doesn't return NULLs it returns
error pointers.

Fixes: b461dfc4 ("blk-mq: add the blk_mq_alloc_disk APIs")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/YMyjci35WBqrtqG+@mwanda


Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Zhong Jinghua <zhongjinghua@huawei.com>
Signed-off-by: Li Nan <linan122@huawei.com>
(cherry picked from commit 5b212263)