kernel/fs: Add support for setting custom VMCaches in vnodes.

This adds one (private) VFS function, and checks in all usages of
the vnode->cache as a VMVnodeCache that it really is one. (Generic
usages, for the moment just the ReleaseRef() calls in vnode
destruction, are intentionally not touched.)

This will be used by ramfs to set the cache from its own,
so that map_file() calls on a ramfs can work.

headers/private/kernel/vfs.h

@@ -112,6 +112,7 @@ status_t	vfs_synchronous_io(io_request* request,
 				void* cookie);
 status_t	vfs_get_vnode_cache(struct vnode *vnode, struct VMCache **_cache,
 				bool allocate);
+status_t	vfs_set_vnode_cache(struct vnode *vnode, struct VMCache *_cache);
 status_t	vfs_get_file_map(struct vnode *vnode, off_t offset, size_t size,
 				struct file_io_vec *vecs, size_t *_count);
 status_t	vfs_get_fs_node_from_path(fs_volume *volume, const char *path,