NFSD: don't start nfsd if sv_permsocks is empty

NFSD: don't start nfsd if sv_permsocks is empty

Previously, while trying to create a server instance, if no
listening sockets were present then default parameter udp
and tcp listeners were created. It's unclear what purpose
was of starting these listeners were and how this could have
been triggered by the userland setup. This patch proposed
to ensure the reverse that we never end in a situation where
no listener sockets are created and we are trying to create
nfsd threads.

The problem it solves is: when nfs.conf only has tcp=n (and
nothing else for the choice of transports), nfsdctl would
still start the server and create udp and tcp listeners.

Signed-off-by: Olga Kornievskaia <okorniev@redhat.com>
Reviewed-by: NeilBrown <neil@brown.name>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>