xen: disable preemption during tlb flush

Various places in the kernel flush the tlb even though preemption doens't
guarantee the tlb flush is happening on any particular CPU.  In many cases
this doesn't seem to matter, so don't make a fuss about it.

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>