rhashtable: Make rhashtable_init params argument const

This patch marks the rhashtable_init params argument const as
there is no reason to modify it since we will always make a copy
of it in the rhashtable.

This patch also fixes a bug where we don't actually round up the
value of min_size unless it is less than HASH_MIN_SIZE.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>