LU-7039 tgt: Delete txn_callback correctly in tgt_init()

LU-7039 tgt: Delete txn_callback correctly in tgt_init()

txn_callback should only be deleted after initialization.

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: Ifdfabe6439c1413d02782d0dfe7a14d6b82ed0df
Reviewed-on: http://review.whamcloud.com/16797
Tested-by: Jenkins
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>