LU-7729 target: fix process_req_last_xid() return value

LU-7729 target: fix process_req_last_xid() return value

process_req_last_xid() returns ptlrpc_error() on error, which
actually returns 0 to caller mistankely.

Test-Parameters: envdefinitions=ONLY=failover_ost \
clientcount=4 osscount=2 mdscount=2 mdtcount=1 \
austeroptions=-R failover=true iscsi=1 \
testlist=recovery-mds-scale

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I136a8ef153a3ea08dcbf05e11fb412e31947be20
Reviewed-on: http://review.whamcloud.com/18245
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>