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.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #1707
Issue #1987
Issue #1891