dm thin: fix crash by initializing thin device's refcount and completion earlier

Commit 80e96c54 ("dm thin: do not allow thin device activation
while pool is suspended") delayed the initialization of a new thin
device's refcount and completion until after this new thin was added
to the pool's active_thins list and the pool lock is released.  This
opens a race with a worker thread that walks the list and calls
thin_get/put, noticing that the refcount goes to 0 and calling
complete, freezing up the system and giving the oops below:

 kernel: BUG: unable to handle kernel NULL pointer dereference at           (null)
 kernel: IP: [<ffffffff810d360b>] __wake_up_common+0x2b/0x90

 kernel: Call Trace:
 kernel: [<ffffffff810d3683>] __wake_up_locked+0x13/0x20
 kernel: [<ffffffff810d3dc7>] complete+0x37/0x50
 kernel: [<ffffffffa0595c50>] thin_put+0x20/0x30 [dm_thin_pool]
 kernel: [<ffffffffa059aab7>] do_worker+0x667/0x870 [dm_thin_pool]
 kernel: [<ffffffff816a8a4c>] ? __schedule+0x3ac/0x9a0
 kernel: [<ffffffff810b1aef>] process_one_work+0x14f/0x400
 kernel: [<ffffffff810b206b>] worker_thread+0x6b/0x490
 kernel: [<ffffffff810b2000>] ? rescuer_thread+0x260/0x260
 kernel: [<ffffffff810b6a7b>] kthread+0xdb/0x100
 kernel: [<ffffffff810b69a0>] ? kthread_create_on_node+0x170/0x170
 kernel: [<ffffffff816ad7ec>] ret_from_fork+0x7c/0xb0
 kernel: [<ffffffff810b69a0>] ? kthread_create_on_node+0x170/0x170

Set the thin device's initial refcount and initialize the completion
before adding it to the pool's active_thins list in thin_ctr().

Signed-off-by: Marc Dionne <marc.dionne@your-file-system.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>