multipath: handle offlined paths

multipath: handle offlined paths

The kernel does not allow multipath to load tables containing offline
devices. Because of this, if you try add a path to a multipath device with
an offline path, the multipathd will continually, retry and fail to reload
the table. I've limited the retries to three to avoid livelocking.

Also, if you map included a offline path, multipath was crashing because
it couldn't get the required sysfs information before calling get_state().
It now checks for this in multipath, like it does in multipathd

Lastly, multipathd would keep reprinting the last checker message for
offlined paths, instead of something useful. It now prints a "path offline"
message.

Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>