[TCP]: TCP Compound quad root function

The original code did a 64 bit divide directly, which won't work on
32 bit platforms.  Rather than doing a 64 bit square root twice,
just implement a 4th root function in one pass using Newton's method.

Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>