multipathd: use 0 for initial pg if there are no priority groups

multipathd: use 0 for initial pg if there are no priority groups

dm-multipath now accommodates a DM table that doesn't have any priority
groups, see: http://git.kernel.org/linus/a490a07a67b7a37

That kernel change imposes that multipathd must pass 0 for the initial
priority group if there aren't any priority groups (currently passes 1).

There is no need to check the multipath target version to decide whether
to pass an initial_pg of 0 or 1 because until recently dm-multipath
never accommodated a DM table with no priority groups.

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