nfsd: fix unused "nn" variable warning in free_client()

If CONFIG_LOCKDEP is disabled, then there would be a warning like this:

  CC [M]  fs/nfsd/nfs4state.o
fs/nfsd/nfs4state.c: In function ‘free_client’:
fs/nfsd/nfs4state.c:1051:19: warning: unused variable ‘nn’ [-Wunused-variable]

So, let's add "maybe_unused" tag to this variable.

Reported-by: Toralf Förster <toralf.foerster@gmx.de>
Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>