random32: add prandom_u32_max and convert open coded users

Many functions have open coded a function that returns a random
number in range [0,N-1]. Under the assumption that we have a PRNG
such as taus113 with being well distributed in [0, ~0U] space,
we can implement such a function as uword t = (n*m')>>32, where
m' is a random number obtained from PRNG, n the right open interval
border and t our resulting random number, with n,m',t in u32 universe.

Lets go with Joe and simply call it prandom_u32_max(), although
technically we have an right open interval endpoint, but that we
have documented. Other users can further be migrated to the new
prandom_u32_max() function later on; for now, we need to make sure
to migrate reciprocal_divide() users for the reciprocal_divide()
follow-up fixup since their function signatures are going to change.

Joint work with Hannes Frederic Sowa.

Cc: Jakub Zawadzki <[email protected]>
Cc: Eric Dumazet <[email protected]>
Cc: [email protected]
Signed-off-by: Hannes Frederic Sowa <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

net/packet/af_packet.c

@@ -1289,7 +1289,7 @@ static unsigned int fanout_demux_rnd(struct packet_fanout *f,
 				     struct sk_buff *skb,
 				     unsigned int num)
 {
-	return reciprocal_divide(prandom_u32(), num);
+	return prandom_u32_max(num);
 }
 
 static unsigned int fanout_demux_rollover(struct packet_fanout *f,