1. If the intention is to coalesce requests 'prev' and 'req' then we
have to ensure at least that we have a layout starting at
req_offset(prev).
2. If we're only requesting a minimal layout of length desc->pg_count,
we need to test the length actually returned by the server before
we allow the coalescing to occur.
3. We need to deal correctly with (pgio->lseg == NULL)
4. Fixup the test guarding the pnfs_update_layout.
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
