x86/MCE/AMD: Fix memory leak when threshold_create_bank() fails

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

x86/MCE/AMD: Fix memory leak when threshold_create_bank() fails

In mce_threshold_create_device(), if threshold_create_bank() fails, the

previously allocated threshold banks array @bp will be leaked because

the call to mce_threshold_remove_device() will not free it.

This happens because mce_threshold_remove_device() fetches the pointer

through the threshold_banks per-CPU variable but bp is written there

only after the bank creation is successful, and not before, when

threshold_create_bank() fails.

Add a helper which unwinds all the bank creation work previously done

and pass into it the previously allocated threshold banks array for

freeing.

[ bp: Massage. ]