nfsd: only register cld pipe notifier when CONFIG_NFSD_V4 is enabled

Otherwise, we get a warning or error similar to this when building with
CONFIG_NFSD_V4 disabled:

    ERROR: "nfsd4_cld_block" [fs/nfsd/nfsd.ko] undefined!

Fix this by wrapping the calls to rpc_pipefs_notifier_register and
..._unregister in another function and providing no-op replacements
when CONFIG_NFSD_V4 is disabled.

Reported-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/netns.h

@@ -31,5 +31,4 @@ struct nfsd_net {
 };
 
 extern int nfsd_net_id;
-extern struct notifier_block nfsd4_cld_block;
 #endif /* __NFSD_NETNS_H__ */