Illumos 4626 - libzfs memleak in zpool_in_use()

Illumos 4626 - libzfs memleak in zpool_in_use()

4626 libzfs memleak in zpool_in_use()
Reviewed by: Tony Nguyen <tony.nguyen@nexenta.com>
Reviewed by: Saso Kiselkov <saso.kiselkov@nexenta.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>

References:
  https://github.com/illumos/illumos-gate/commit/fb13f48
  https://www.illumos.org/issues/4626

Ported-by: kernelOfTruth kerneloftruth@gmail.com
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3563