NFS: Verify server address before invoking in-kernel mount client

Re-order mount option sanity checking slightly to ensure we have a valid
server address *before* trying to do the mountd RPC call.

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