dm: add disk before alloc dax

Offering: HULK
hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I78SWJ


CVE: NA

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In dm_create(), alloc_dev() may trigger panic if alloc_dax() fail since
del_gendisk() will be called with add_disk() wasn't called before.

Call add_disk() before alloc_dax() to avoid it.

Signed-off-by: Li Lingfeng <lilingfeng3@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>
(cherry picked from commit 6601d443)