Add support for force unmounting ZFS filesystems during "zfs rename"
with the -f flag.

Reimplementation of the illumos changeset 13677:a0cbef703c12
2635 'zfs rename -f' to perform force unmount

References:
https://www.illumos.org/issues/2635

PR: kern/164447
Suggested by: Marcelo Araujo <araujo@FreeBSD.org>
Obtained from: illumos (issue #2635)
MFC after: 1 week