LU-11462 osd-zfs: skip sync if object is not dirty

LU-11462 osd-zfs: skip sync if object is not dirty

there is no need to flush whole filesystem if the object is not
dirty or being synced.

in SLOW=yes FSTYPE=zfs sh sanity-benchmark.sh ~24% sync reqs
can be skipped.

Change-Id: I13b15ed646a4603858002e8be25ef5f5f8cd2b9b
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/33276
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Nathaniel Clark <nclark@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>