dm round robin: do not use this_cpu_ptr() without having preemption disabled

Use local_irq_save() to disable preemption before calling
this_cpu_ptr().

Reported-by: Benjamin Block <bblock@linux.vnet.ibm.com>
Fixes: b0b477c7 ("dm round robin: use percpu 'repeat_count' and 'current_path'")
Cc: stable@vger.kernel.org # 4.6+
Suggested-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>