NFSD: Create PF_INET6 listener in write_ports

Try to create a PF_INET6 listener for NFSD, if IPv6 is enabled in the
kernel.

Make sure nfsd_serv's reference count is decreased if
__write_ports_addxprt() failed to create a listener.  See
__write_ports_addfd().

Our current plan is to rely on rpc.nfsd to create appropriate IPv6
listeners when server-side NFS/IPv6 support is desired.  Legacy
behavior, via the write_threads or write_svc kernel APIs, will remain
the same -- only IPv4 listeners are created.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
[bfields@citi.umich.edu: Move error-handling code to end]
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>