nfsd: Fix up nfsd to ensure that timeout errors don't result in ESTALE

If the underlying filesystem times out, then we want knfsd to return
NFSERR_JUKEBOX/DELAY rather than NFSERR_STALE.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>