OpenZFS 6940 - Cannot unlink directories when over quota

OpenZFS 6940 - Cannot unlink directories when over quota

From user perspective, I would expect that ZFS is always able
to remove files and directories even when the quota is exceeded.

Authored by: Simon Klinkert <simon.klinkert@gmail.com>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Ported-by: kernelOfTruth kerneloftruth@gmail.com
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
OpenZFS-issue: https://www.illumos.org/issues/6940
OpenZFS-issue: https://www.illumos.org/issues/6334
OpenZFS-commit: https://github.com/illumos/illumos-gate/commit/9918916
Closes #5044