arch/x86/kernel: Add missing spin_unlock

Add a spin_unlock missing on the error path.  The locks and unlocks are
balanced in other functions, so it seems that the same should be the case
here.

The semantic match that finds this problem is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@@
expression E1;
@@

* spin_lock(E1,...);
  <+... when != E1
  if (...) {
    ... when != E1
*   return ...;
  }
  ...+>
* spin_unlock(E1,...);
// </smpl>

Cc: stable@kernel.org
Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>