LU-6386 tgt: don't update client data with smaller transno

LU-6386 tgt: don't update client data with smaller transno

Fix tgt_last_rcvd_update() to don't update transaction number
in client slot with smaller value.

Also patch removes outdated code about ted_lcd == NULL case.
This is not possible now, because lcd is set to NULL only
upon export destroy. This check was needed in past when that
lcd was set to NULL during export disconnect and some activity
was still possible on this export.

Signed-off-by: Mikhail Pershin <mike.pershin@intel.com>
Change-Id: I34717ea91493785beadcf725d49c4c9265b63f7c
Reviewed-on: http://review.whamcloud.com/14113
Tested-by: Jenkins
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: wangdi <di.wang@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>