LU-6719 osd-zfs: Ignore EEXIST during object init

LU-6719 osd-zfs: Ignore EEXIST during object init

ZFS can return EEXIST if object exists but is being destroyed.

Specifically see dnode_hold_impl()

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: Id99b406b2f02a1337b9f1566fba30dbced755d5d
Reviewed-on: http://review.whamcloud.com/18054
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Jenkins
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>