MFV r302650: 6940 Cannot unlink directories when over quota

MFV r302650: 6940 Cannot unlink directories when over quota

illumos/illumos-gate@99189164df06057fb968ca7be701bb1a0d5da8c9
https://github.com/illumos/illumos-gate/commit/99189164df06057fb968ca7be701bb1a0d5da8c9

https://www.illumos.org/issues/6940
  Similar to #6334, but this time with empty directories:
  $ zfs create tank/quota
  $ zfs set quota=10M tank/quota
  $ zfs snapshot tank/quota@snap1
  $ zfs set mountpoint=/mnt/tank/quota tank/quota
  $ mkdir /mnt/tank/quota/dir # create an empty directory
  $ mkfile 11M /mnt/tank/quota/11M
  /mnt/tank/quota/11M: initialized 9830400 of 11534336 bytes: Disc quota exceeded
  $ rmdir /mnt/tank/quota/dir # now unlink the empty directory
  rmdir: directory "/mnt/tank/quota/dir": Disc quota exceeded
  From user perspective, I would expect that ZFS is always able to remove files
  and directories even when the quota is exceeded.

Reviewed by: Dan McDonald <danmcd@omniti.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Simon Klinkert <simon.klinkert@gmail.com>
MFC after:	2 weeks