libnetfs: add netfs_make_node_alloc to allocate fat nodes

libnetfs has two kind of nodes, struct node and struct netnode.
struct node is used to store libnetfs specific data, while struct
netnode contains user supplied data.  Previously, both objects were
allocated separatly, and a pointer from the node to the netnode
provided a mapping from the former to the latter.

Provide a function netfs_make_node_alloc that allocates both nodes in
a contiguous region.

This reduces the memory allocation overhead when creating nodes.  It
also makes the relation between node and netnode a simple offset
calculation.  Provide two functions to compute the netnode address
from the node address and vice-versa.

Most notably, this makes implementing a cache on top of libnetfs
easier.  Auxiliary data for the cache can be stored in the
user-defined netnode, and the fat node can be used as the value.

* libnetfs/make-node.c (init_node): Move initialization here.
(netfs_make_node): Use init_node.
(netfs_make_node_alloc): New function to allocate fat nodes.
* libnetfs/netfs.h (netfs_make_node_alloc): New declaration.
(_netfs_sizeof_struct_node): Likewise.
(netfs_node_netnode): Compute netnode address from node address.
(netfs_netnode_node): And vice-versa.
* libnetfs/init-init.c (_netfs_sizeof_struct_node): New variable.

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