MFZoL: Add -vnP support to 'zfs send' for bookmarks

zfsonlinux/zfs@835db58592d7d947e5818eb7281882e2a46073e0

We have long supported estimating a size of an incremental stream from a
snapshot. We should do the same for bookmarks as well.

Obtained from: ZoL
Author: loli10K <ezomori.nozomu@gmail.com>
MFC after: 3 days