codel: use Newton method instead of sqrt() and divides

As Van pointed out, interval/sqrt(count) can be implemented using
multiplies only.

http://en.wikipedia.org/wiki/Methods_of_computing_square_roots#Iterative_methods_for_reciprocal_square_roots

This patch implements the Newton method and reciprocal divide.

Total cost is 15 cycles instead of 120 on my Corei5 machine (64bit
kernel).

There is a small 'error' for count values < 5, but we don't really care.

I reuse a hole in struct codel_vars :
 - pack the dropping boolean into one bit
 - use 31bit to store the reciprocal value of sqrt(count).

Suggested-by: Van Jacobson <van@pollere.net>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Dave Taht <dave.taht@bufferbloat.net>
Cc: Kathleen Nichols <nichols@pollere.com>
Cc: Tom Herbert <therbert@google.com>
Cc: Matt Mathis <mattmathis@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Nandita Dukkipati <nanditad@google.com>
Cc: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>