nfsd: make nfsd_filecache_wq variable static

Fix sparse warning:

fs/nfsd/filecache.c:55:25: warning:
	symbol 'nfsd_filecache_wq' was not declared. Should it be static?
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/filecache.c

@@ -52,7 +52,7 @@ struct nfsd_fcache_disposal {
 	struct rcu_head rcu;
 };
 
-struct workqueue_struct *nfsd_filecache_wq __read_mostly;
+static struct workqueue_struct *nfsd_filecache_wq __read_mostly;
 
 static struct kmem_cache		*nfsd_file_slab;
 static struct kmem_cache		*nfsd_file_mark_slab;