zfs: Add option for forcible unmounting dataset while receiving snapshot.

Currently when the dataset is in use we can't receive snapshots.

zfs send test/1@asd | zfs recv -FM test/2
cannot unmount '/test/2': Device busy

This commits add option 'M' which attempts to forcibly unmount the
dataset. Thanks to this we can enforce receiving snapshots in a
single step.

Note that this functionality is not supported on Linux because the
VFS will prevent active mounted filesystems from being unmounted,
even with the force option. This is the intended VFS behavior.

Discussed-with: Pawel Jakub Dawidek <pjd@FreeBSD.org>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Allan Jude <allanjude@freebsd.org>
Differential Revision: https://reviews.freebsd.org/D22306

openzfs/zfs@a57d3d45d6efdff935421e2ef3f97e3dc089d93d