netfilter: remove NF_NAT_RANGE_PROTO_RANDOM support

Historically this was net_random() based, and was then converted to
a hash based algorithm (private boot seed + hash of endpoint addresses)
due to concerns of leaking net_random() bits.

RANDOM_FULLY mode was added later to avoid problems with hash
based mode (see commit 34ce3240,
"netfilter: nf_nat: add full port randomization support" for details).

Just make prandom_u32() the default search starting point and get rid of
->secure_port() altogether.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>