nfsd: use i_wrlock instead of rcu for nfsdfs i_private

synchronize_rcu() gets called multiple times each time a client is
destroyed.  If the laundromat thread has a lot of clients to destroy,
the delay can be noticeable.  This was causing pynfs test RENEW3 to
fail.

We could embed an rcu_head in each inode and do the kref_put in an rcu
callback.  But simplest is just to take a lock here.

(I also wonder if the laundromat thread would be better replaced by a
bunch of scheduled work or timers or something.)
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfsctl.c

@@ -1215,11 +1215,9 @@ static void clear_ncl(struct inode *inode)
 	struct nfsdfs_client *ncl = inode->i_private;
 
 	inode->i_private = NULL;
-	synchronize_rcu();
 	kref_put(&ncl->cl_ref, ncl->cl_release);
 }
 
-
 static struct nfsdfs_client *__get_nfsdfs_client(struct inode *inode)
 {
 	struct nfsdfs_client *nc = inode->i_private;
@@ -1233,9 +1231,9 @@ struct nfsdfs_client *get_nfsdfs_client(struct inode *inode)
 {
 	struct nfsdfs_client *nc;
 
-	rcu_read_lock();
+	inode_lock_shared(inode);
 	nc = __get_nfsdfs_client(inode);
-	rcu_read_unlock();
+	inode_unlock_shared(inode);
 	return nc;
 }
 /* from __rpc_unlink */