pNFS/flexfiles: Report ENETDOWN as a connection error

If the client should see an ENETDOWN when trying to connect to the data
server, it might still be able to talk to the metadata server through
another NIC. If so, report the error.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Tested-by: Jeff Layton <jlayton@kernel.org>
Acked-by: Chuck Lever <chuck.lever@oracle.com>