nfs-utils: don't use signals to shut down nfs server.

nfs-utils: don't use signals to shut down nfs server.

Since Linux v2.4 it has been possible to stop all NFS server by running

   rpc.nfsd 0

i.e.  by requesting that zero threads be running.  This is preferred as
it doesn't risk killing some other process which happens to be called
"nfsd".

Since Linux v6.6 - and other stable kernels to which

  Commit: 390390240145 ("nfsd: don't allow nfsd threads to be
  signalled.")

has been backported - sending a signal no longer works to stop nfs server
threads.

This patch changes the nfsserver script to use "rpc.nfsd 0" to stop
server threads.

Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>