mm: allow preemption in apply_to_pte_range

Impact: allow preemption in apply_to_pte_range updates to init_mm

Preemption is now allowed for lazy mmu mode, so don't disable
it for the inner loop of apply_to_pte_range.  This only applies
when doing updates to init_mm; user pagetables are still modified
under the pte lock, so preemption is disabled anyway.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>