The runtime/map* functions have several weaknesses w.r.t. hashing
that we should improve.

1) HASH_TABLE_BITS/SIZE should probably be a function of MAXMAPENTRIES.
2) str_hash() should be more hashy.  It looks rather wimpy.
3) With deliberate hash-collision-inducing data conceivably fed to stap,
   we should investigate salting the map FOO_hash functions with some
   runtime-random value, such as a few bytes collected at startup time with
   get_random_bytes(), as in

static unsigned long stap_hash_seed; /* init during module startup */
static unsigned int int64_hash (const int64_t v)
{
        return (unsigned int)hash_long (((unsigned long)v) ^ stap_hash_seed,
                                        HASH_TABLE_BITS);
}
/* and similarly for str_hash */