In current pnfs tree, all the layouts set mds_offset in their
.write_pagelist member.
mds_offset is only used by generic layer and should be handled by it.

This patch is for upstream. It is needed in this -rc series to fix a
bug in objects layout_commit.

I'll send patches for objects and blocks to be
squashed into current pnfs tree.

TODO: It looks like the read path needs the same patch.
Signed-off-by: NBoaz Harrosh <bharrosh@panasas.com>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>