zfs filesystem skipped by df -h

On full pool when pool root filesystem references very few bytes,
the f_blocks returned to statvfs is 0 but should be at least 1.

Reviewed by: Tom Caputi <tcaputi@datto.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Paul Zuchowski <pzuchowski@datto.com>
Closes #8253
Closes #8254