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 openzfs#1707
Issue openzfs#1987
Issue openzfs#1891

module/zfs/spa_misc.c

@@ -1552,7 +1552,9 @@ dva_get_dsize_sync(spa_t *spa, const dva_t *dva)
 
 	if (asize != 0 && spa->spa_deflate) {
 		vdev_t *vd = vdev_lookup_top(spa, DVA_GET_VDEV(dva));
-		dsize = (asize >> SPA_MINBLOCKSHIFT) * vd->vdev_deflate_ratio;
+		if (vd != NULL)
+			dsize = (asize >> SPA_MINBLOCKSHIFT) *
+			    vd->vdev_deflate_ratio;
 	}
 
 	return (dsize);