dm mpath: rdac fix init race

Re-order the initialisation of dm-rdac to avoid registering the hw
handler before the workqueue has been initialised. Closes a race
that would potentially give an oops.

Signed-off-by: Bryn M. Reeves <breeves@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>