nfs: Properly initialize server->writeback

Atomic types should better be initialized with atomic_long_set() instead
of relying on zeroing done by kzalloc(). Clean this up.
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jan Kara <jack@suse.cz>
Acked-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/client.c

@@ -994,6 +994,8 @@ struct nfs_server *nfs_alloc_server(void)
 
 	server->change_attr_type = NFS4_CHANGE_TYPE_IS_UNDEFINED;
 
+	atomic_long_set(&server->writeback, 0);
+
 	ida_init(&server->openowner_id);
 	ida_init(&server->lockowner_id);
 	pnfs_init_server(server);