r320062 introduced a bug when doing NFSv4.1 mounts against some non-FreeBSD servers.

r320062 used nm_rsize, nm_wsize to set the maximum request/response sizes for
the NFSv4.1 session. If rsize,wsize are not specified as options, the
value of nm_rsize, nm_wsize is 0 at session creation, resulting in
values for request/response that are too small.
This patch fixes the problem. A workaround is to specify rsize=N,wsize=N
mount options explicitly, so they are set before session creation.
This bug only affects NFSv4.1 mounts against some non-FreeBSD servers.

MFC after: 1 week