nfsd4: BUG_ON(!is_spin_locked()) no good on UP kernels

Most frequent symptom was a BUG triggering in expire_client, with the
server locking up shortly thereafter.

Introduced by 508dc6e1 "nfsd41:
free_session/free_client must be called under the client_lock".

Cc: stable@kernel.org
Cc: Benny Halevy <bhalevy@tonian.com>
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4state.c

@@ -900,7 +900,7 @@ static void free_session(struct kref *kref)
 	struct nfsd4_session *ses;
 	int mem;
 
-	BUG_ON(!spin_is_locked(&client_lock));
+	lockdep_assert_held(&client_lock);
 	ses = container_of(kref, struct nfsd4_session, se_ref);
 	nfsd4_del_conns(ses);
 	spin_lock(&nfsd_drc_lock);
@@ -1080,7 +1080,7 @@ static struct nfs4_client *alloc_client(struct xdr_netobj name)
 static inline void
 free_client(struct nfs4_client *clp)
 {
-	BUG_ON(!spin_is_locked(&client_lock));
+	lockdep_assert_held(&client_lock);
 	while (!list_empty(&clp->cl_sessions)) {
 		struct nfsd4_session *ses;
 		ses = list_entry(clp->cl_sessions.next, struct nfsd4_session,