MAINTAINERS: Add a bugzilla link for NFSD

I recently found out about B: for noting where subsystem bugs can be
filed. I do pay attention to bugzilla.kernel.org. The Linux NFS
community has decided to steer kernel NFS issues to the kernel.org
bugzilla instead of bugzilla.linux-nfs.org.

Remove the W: entry; the sourceforge information is stale and
unmaintained.

Cc: Thorsten Leemhuis <linux@leemhuis.info>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>