zfs: add option for forcible unmounting dataset while receiving snapshot.
Currently when the dataset is in use we can't receive snapshot.
zfs send test/1@asd | zfs recv -FM test/2
cannot unmount '/test/2': Device busy
This commits add option 'M' which forcible unmounting the dataset.
Thanks to to that we can enforce receiving snapshot in single step.
Discussed with: pjd
If we think that this is a good idea I can submit pull request to
the OpenZFS as well.