SUNRPC: Allow RPC calls to return ETIMEDOUT instead of EIO

On occasion, it is useful for the NFS layer to distinguish between
soft timeouts and other EIO errors due to (say) encoding errors,
or authentication errors.

The following patch ensures that the default behaviour of the RPC
layer remains to return EIO on soft timeouts (until we have
audited all the callers).

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