xen: fold xen_sysexit into xen_iret

xen_sysexit and xen_iret were doing essentially the same thing.  Rather
than having a separate implementation for xen_sysexit, we can just strip
the stack back to an iret frame and jump into xen_iret.  This removes
a lot of code and complexity - specifically, another critical region.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>