Fix in check_filesystem()

Fix the code in case of missing snapshots. Previously the check was in
a conditional that would be executed if the filesystem had snapshots.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes #14735