NFS: make struct nlmclnt_fl_close_lock_ops static

The structure nlmclnt_fl_close_lock_ops s local to the source and does
not need to be in global scope, so make it static.

Cleans up sparse warning:
fs/nfs/nfs3proc.c:876:33: warning: symbol 'nlmclnt_fl_close_lock_ops' was not
declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>