dm mpath: split and rename activate_path() to prepare for its expanded use

activate_path() is renamed to activate_path_work() which now calls
activate_or_offline_path().  activate_or_offline_path() will be used
by the next commit.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>