Fix a bug in the device pager code that can trigger an assertion

Fix a bug in the device pager code that can trigger an assertion
in devfs if a particular race condition is hit in the device pager
code.

This was a side effect of change 227530 which changed the device
pager interface to call a new destructor routine for the cdev.
That destructor routine, old_dev_pager_dtor(), takes a VM object
handle.

The object handle is cast to a struct cdev *, and passed into
dev_rel().

That works in most cases, except the case in cdev_pager_allocate()
where there is a race condition between two threads allocating an
object backed by the same device.  The loser of the race
deallocates its object at the end of the function.

The problem is that before inserting the object into the
dev_pager_object_list, the object's handle is changed from the
struct cdev pointer to the object's own address.  This is to avoid
conflicts with the winner of the race, which already inserted an
object in the list with a handle that is a pointer to the same cdev
structure.

The object is then passed to vm_object_deallocate(), and eventually
makes its way down to old_dev_pager_dtor().  That function passes
the handle pointer (which is actually a VM object, not a struct
cdev as usual) into dev_rel().  dev_rel() decrements the reference
count in the assumed struct cdev (which happens to be 0), and
that triggers the assertion in dev_rel() that the reference count
is greater than or equal to 0.

The fix is to add a cdev pointer to the VM object, and use that
pointer when calling the cdev_pg_dtor() routine.

vm_object.h:	Add a struct cdev pointer to the VM object
		structure.

device_pager.c:	In cdev_pager_allocate(), populate the new cdev
		pointer.

		In dev_pager_dealloc(), use the new cdev pointer
		when calling the object's cdev_pg_dtor() routine.

Reviewed by:	kib
Sponsored by:	Spectra Logic Corporation
MFC after:	1 week