LU-8695 target: use -1 as an offset to declare write

LU-8695 target: use -1 as an offset to declare write

at the end of recovery or filesystem setup the number of clients
may increase significantly. this can lead to underestimated space
or credits reserved.

Change-Id: Id4f3755dc481f8a29a1a2a673c26d64d12f7dbf0
Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-on: http://review.whamcloud.com/23082
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>