LU-7713 osd: osd-zfs should serialize destroy vs. others

LU-7713 osd: osd-zfs should serialize destroy vs. others

otherwise we can get unexpected EEXIST from DMU at any time.
sanityn/91 hits this regularly.

Change-Id: I948413a0689f1ceae7f073a1e33adef023eb274c
Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-on: http://review.whamcloud.com/18155
Tested-by: Jenkins
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>