dm mpath: cleanup reinstate_path() et al based on code review

fail_path() will print a "Failing path ..." message but reinstate_path()
doesn't print a "Reinstating path ...".  Add that message to
reinstate_path() to add symmetry and aid system debugging.

Remove reinstate_path()'s check for the path_selector providing
.reinstate_path hook.  All path selectors provide this and any future
ones must too.

activate_path() calls pg_init_done() with SCSI_DH_DEV_OFFLINED but
pg_init_done() doesn't expicitly handle it in its swicth statement.  Add
SCSI_DH_DEV_OFFLINED to the default case.

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