Set 'zfs_expire_snapshot=0' to disable auto-unmount

There are cases where it's desirable that auto-mounted snapshots
not expire after a fixed duration. They should be unmounted only
when the filesystem they are a snapshot of is unmounted.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>