OpenZFS 6404 - zvol_swap_006_pos can occasionally fail due to swaplen being < 16

OpenZFS 6404 - zvol_swap_006_pos can occasionally fail due to swaplen being < 16

Authored by: Matthew Ahrens <mahrens@delphix.com>
6405 zvol test setup is non deterministic
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: John Kennedy <john.kennedy@delphix.com>
Reviewed by: Will Andrews <will@freebsd.org>
Approved by: Dan McDonald <danmcd@omniti.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: George Melikov <mail@gmelikov.ru>
OpenZFS-issue: https://www.illumos.org/issues/6404
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/24e268f
Closes #5792

Porting notes:
- Converted zfs to $ZFS until OpenZFS 7290 is ported.
  openzfs/openzfs@1d32ba6