always_null member in jffs2 structs

Question

Looking through the jffs2 code in Linux, I noticed that first member in two structs in xattr.h is the same and it is void *always_null. Here is one of the structs:

struct jffs2_xattr_datum
{
    void *always_null;
    struct jffs2_raw_node_ref *node;
    uint8_t class;
    uint8_t flags;
    uint16_t xprefix;

    struct list_head xindex;
    atomic_t refcnt;
    uint32_t xid;
    uint32_t version;

    uint32_t data_crc;
    uint32_t hashkey;
    char *xname;
    uint32_t name_len;
    char *xvalue;
    uint32_t value_len;
};

I am wondering why this member is created and what it is used for.