md/raid1: fix memory leak in raid1_run()

In the Linux kernel, the following vulnerability has been resolved:

md/raid1: fix memory leak in raid1_run()

raid1_run() calls setup_conf() which registers a thread via

md_register_thread(). If raid1_set_limits() fails, the previously

registered thread is not unregistered, resulting in a memory leak

of the md_thread structure and the thread resource itself.

Add md_unregister_thread() to the error path to properly cleanup

the thread, which aligns with the error handling logic of other paths

in this function.

Compile tested only. Issue found using a prototype static analysis tool

and code review.