Bug#648635: May be it is out of nfs-utils' control

["WHR" <whr@rivoreo.one>]

I found a workaround.

By using option '--host' in rpc.nfsd(8), it will creating the socket from user
space, then passing the file descriptor to kernel via /proc/fs/nfsd/portlist;
thus bypasses the previously mentioned socket creation code in kernel.

So to listen on IPv4 only, pass option '--host=0.0.0.0' to rpc.nfsd(8). For
Debian Bullseye and early releases, add this option to 'RPCNFSDOPTS' variable
of file /etc/default/nfs-kernel-server; for example:
	RPCNFSDOPTS="--no-udp --host=0.0.0.0"
For later releases, simply set the 'host' directive under '[nfsd]' section.

Now I have:
	# netstat -nl | grep -F :2049
	tcp        0      0 0.0.0.0:2049            0.0.0.0:*               LISTEN