If the server tells us that a pNFS layout is not available for a
specific file, then we should not keep pounding it with further
layoutget requests.

Fixes: 183d9e7b ("pnfs: rework LAYOUTGET retry handling")
Signed-off-by: NTrond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: NAnna Schumaker <Anna.Schumaker@Netapp.com>