dm round robin: use percpu 'repeat_count' and 'current_path'

Now that dm-mpath core is lockless in the per-IO fast path it is
critical, for performance, to have the .select_path hook
(rr_select_path) also be as lockless as possible.

The new percpu members of 'struct selector' allow for lockless support
of 'repeat_count' governed repeat use of a previously selected path.  If
a path fails while it is 'current_path' the worst case is concurrent IO
might be mapped to the failed path until the .fail_path hook
(rr_fail_path) is called.

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