Revert "xen/smp: Fix CPU online/offline bug triggering a BUG: scheduling while atomic."

This reverts commit 41bd956d.

The fix is incorrect and not appropiate for the latest kernels.
In fact it _causes_ the BUG: scheduling while atomic while
doing vCPU hotplug.

Suggested-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>