LU-7414 target: do not share update and rdbuf

LU-7414 target: do not share update and rdbuf

Redefine lu_rdbuf structure to simplify the rdbuf
allocation in out_read().

And also move tti_u.rdbuf out of tgt_thread_info
union, otherwise rdbuf and update will share
the same memory and cause corruption, see out_read().

Signed-off-by: Di Wang <di.wang@intel.com>
Change-Id: Idb0f5af1b00fd5fd15ebc8742aa60d9a43df0a8a
Reviewed-on: http://review.whamcloud.com/17129
Tested-by: Jenkins
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>