Align code with Illumos

Align code with Illumos

Align code in traverse_visitbp() with that in Illumos in preparation for
applying Illumos-5694.

No functional change: use a temporary variable pd to replace multiple
occurrences of td->td_pfd.  This increases our stack use slightly more
then normal because the function is called recursively.

Signed-off-by: Chris Dunlop <chris@onthe.net.au>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #3230