Silence e2fsck warning in zconfig.sh

When running zconfig.sh test 7 and 8 cause the following warning to
be printed to the console. It's caused because we're snapshoting a
mounted ext2 filesystem which is not in a 'clean' state. This is
to be expected since we have no guarentees about the on-disk
consistency of the filesystem.

EXT2-fs warning: mounting unchecked fs, running e2fsck is recommended

To silence the warning and preserve the intent of these test cases
they have been updated to unmount the filesystem prior to snapshoting
them. This ensures the ext2 filesystem is in a consistent state
when the snapshot is taken.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ned Bass <bass6@llnl.gov>
Closes #1972