dm: fix NULL pointer when clone_and_map_rq returns !DM_MAPIO_REMAPPED

When stacking request-based DM on blk_mq device, request cloning and
remapping are done in a single call to target's clone_and_map_rq().
The clone is allocated and valid only if clone_and_map_rq() returns
DM_MAPIO_REMAPPED.

The "IS_ERR(clone)" check in map_request() does not cover all the
!DM_MAPIO_REMAPPED cases that are possible (E.g. if underlying devices
are not ready or unavailable, clone_and_map_rq() may return
DM_MAPIO_REQUEUE without ever having established an ERR_PTR).  Fix this
by explicitly checking for a return that is not DM_MAPIO_REMAPPED in
map_request().

Without this fix, DM core may call setup_clone() for a NULL clone
and oops like this:

   BUG: unable to handle kernel NULL pointer dereference at 0000000000000068
   IP: [<ffffffff81227525>] blk_rq_prep_clone+0x7d/0x137
   ...
   CPU: 2 PID: 5793 Comm: kdmwork-253:3 Not tainted 4.0.0-nm #1
   ...
   Call Trace:
    [<ffffffffa01d1c09>] map_tio_request+0xa9/0x258 [dm_mod]
    [<ffffffff81071de9>] kthread_worker_fn+0xfd/0x150
    [<ffffffff81071cec>] ? kthread_parkme+0x24/0x24
    [<ffffffff81071cec>] ? kthread_parkme+0x24/0x24
    [<ffffffff81071fdd>] kthread+0xe6/0xee
    [<ffffffff81093a59>] ? put_lock_stats+0xe/0x20
    [<ffffffff81071ef7>] ? __init_kthread_worker+0x5b/0x5b
    [<ffffffff814c2d98>] ret_from_fork+0x58/0x90
    [<ffffffff81071ef7>] ? __init_kthread_worker+0x5b/0x5b

Fixes: e5863d9a ("dm: allocate requests in target when stacking on blk-mq devices")
Reported-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org # 4.0+