dm: fix rq_end_stats() NULL pointer in dm_requeue_original_request()

dm: fix rq_end_stats() NULL pointer in dm_requeue_original_request()

commit 98dbc9c6c61698792e3a66f32f3bf066201d42d7 upstream.

An "old" (.request_fn) DM 'struct request' stores a pointer to the
associated 'struct dm_rq_target_io' in rq->special.

dm_requeue_original_request(), previously named
dm_requeue_unmapped_original_request(), called dm_unprep_request() to
reset rq->special to NULL.  But rq_end_stats() would go on to hit a NULL
pointer deference because its call to tio_from_request() returned NULL.

Fix this by calling rq_end_stats() _before_ dm_unprep_request()

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Fixes: e262f34741 ("dm stats: add support for request-based DM devices")
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>