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>

diff --cc fs/nfsd/netns.h
Simple merge

diff --cc fs/nfsd/nfs4recover.c
Simple merge

diff --cc fs/nfsd/nfsctl.c
Simple merge

diff --cc fs/nfsd/nfsd.h
Simple merge