Issue 1707

Handle vdev_lookup_top() failure in dva_get_dsize_sync()
    
The dva_get_dsize_sync() function incorrectly assumes that the call
to vdev_lookup_top() cannot fail.  However, the NULL dereference at
clearly shows that under certain circumstances it is possible.  Note
that offset 0x570 (1376) maps as expected to vd->vdev_deflate_ratio.
    
  BUG: unable to handle kernel NULL pointer dereference at 00000570
    
  crash> struct -o vdev
  struct vdev {
       [0] uint64_t vdev_id;
       ... ...
    [1376] uint64_t vdev_deflate_ratio;
    
Given that this can happen this patch add the required error handling.
In the case where vdev_lookup_top() fails assume that no deflation
will occur for the DVA and use the asize.