LU-7918 mdd: no need to declare attr_set at create

LU-7918 mdd: no need to declare attr_set at create

as dt_create() will set the attributes we need. also, we can save
couple dt_record_write() in tgt_txn_start_cb() as one is not needed
with multislot enabled while another is not needed at all.
these changes decrease disk/memory reservation from ~33MB to 6MB.

Change-Id: I65e537dd053eb7b4e4d2cccab41973096072b1db
Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-on: http://review.whamcloud.com/19132
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>