7301 zpool export -f should be able to interrupt file freeing

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Sanjay Nadkarni <sanjay.nadkarni@nexenta.com>
Reviewed by: Saso Kiselkov <saso.kiselkov@nexenta.com>
Reviewed by: John Kennedy <john.kennedy@delphix.com>
Author: Alek Pinchuk <alek@nexenta.com>

Closes #175