LU-12309 osd-zfs: Support disabled project quotas

LU-12309 osd-zfs: Support disabled project quotas

Allow project quotas to be compiled in but disabled in the zpool.
This would be the case for zpools created by pre-0.8.0 ZFS, but then
used with newer ZFS.

Signed-off-by: Nathaniel Clark <nclark@whamcloud.com>
Change-Id: I79c2c4ee3b191dad4150c218b25ced2508062d51
Reviewed-on: https://review.whamcloud.com/34888
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Olaf Faaland-LLNL <faaland1@llnl.gov>
Reviewed-by: Oleg Drokin <green@whamcloud.com>