nfs41: Correct offset for LAYOUTCOMMIT

A client sends offset to MDS as it was seen by DS. As result,
file size after copy is only half of original file size in case
of 2 DS.

Signed-off-by: Vitaliy Gusev <gusev.vitaliy@nexenta.com>
Cc: stable@kernel.org [2.6.39]
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>