dm kcopyd: avoid spin_lock_irqsave from process context

The functions "pop", "push_head", "do_work" can only be called from
process context. Therefore, replace spin_lock_irq{save,restore} with
spin_{lock,unlock}_irq.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>