nfsd4: free_stid can be static

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index 60a2282..c1a6ddf 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -350,7 +350,7 @@
 	return dp;
 }
 
-void free_stid(struct nfs4_stid *s, struct kmem_cache *slab)
+static void free_stid(struct nfs4_stid *s, struct kmem_cache *slab)
 {
 	struct idr *stateids = &s->sc_client->cl_stateids;