inetpeer: use div64_ul() and clamp_val() calculate inet_peer_threshold

In inet_initpeers(), struct inet_peer on IA32 uses 128 bytes in nowdays.
Get rid of the cascade and use div64_ul() and clamp_val() calculate that
will not need to be adjusted in the future as suggested by Eric Dumazet.

Suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Yejune Deng <yejune.deng@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>