md: fix sysfs duplicate file while adding rdev

hulk inclusion
category: bugfix
bugzilla: 188553, https://gitee.com/openeuler/kernel/issues/I6TNFX


CVE: NA

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

rdev->del_work has not been queued to md_rdev_misc_wq and flush_workqueue
will not flush it if tow threads add and remove same device. sysfs might
WARN duplicate filename as below.

    //T1	             //T2
    mdadm write super
			     add success
			     remove
			      unbind_rdev_from_array

    md_ioctl
     flush_workqueue
			      INIT_WORK
                               queue_work
     md_add_new_disk
      duplicate filename dev-xxx

Check if there is any kobj with the same name, and return busy if true.

Fixes: 5792a285 ("md: avoid a deadlock when removing a device from an md array via sysfs")
Signed-off-by: Li Nan <linan122@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>