mm: disable preemption in apply_to_pte_range

Impact: bugfix

Lazy mmu mode needs preemption disabled, so if we're apply to
init_mm (which doesn't require any pte locks), then explicitly
disable preemption.  (Do it unconditionally after checking we've
successfully done the allocation to simplify the error handling.)

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>