dm mpath: fix support for loading scsi_dh modules during table load

The ability to have multipath dynamically attach a scsi_dh, that the user
specified in the multipath table, was broken by commit e8f74a0f ("dm
mpath: eliminate need to use scsi_device_from_queue").

Restore the ability to load, and attach, a particular scsi_dh module if
one is specified (as noticed by checking m->hw_handler_name).

Fixes: e8f74a0f

 ("dm mpath: eliminate need to use scsi_device_from_queue")
Reported-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>