pnfs: check that partial LAYOUTGET return is ignored

Either a bad server reply, or our ignoring of multiple array segments in
a reply, can cause a reply to not meet our requirements.  Ensure
that we ignore such replies.

Signed-off-by: Fred Isaman <iisaman@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>