LU-6160 osd-zfs: Fix refcount_add call

LU-6160 osd-zfs: Fix refcount_add call

Correct the refcount_add in osd-zfs module's osd_fix_new_dnode
function. The variable 'tag' was undefined and caused osd-zfs
to fail builds against zfs packages with debug enabled.

This small change should enable lustre to be built against
zfs packages that have debug enabled.

Test-Parameters: trivial
Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Change-Id: If95f0af6178cf0ea78724658edfaece1ee16a3f1
Reviewed-on: https://review.whamcloud.com/28544
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>