mount.nfs: Don't update extra_opts after text-based negotiation

The umount.nfs command will negotiate the mount options again, so all
that is needed in /etc/mnttab is the original set of options used for
the mount, plus the additional mandatory options like addr=''.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>