dm clone: add bucket_lock_irq/bucket_unlock_irq helpers

Introduce bucket_lock_irq() and bucket_unlock_irq() helpers and use them
in places where it is known that interrupts are enabled.

Signed-off-by: Nikos Tsironis <ntsironis@arrikto.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>