tcp_probe: avoid modulus operation and wrap fix

By rounding up the buffer size to power of 2, several expensive
modulus operations can be avoided.  This patch also solves a bug where
the gap need when ring gets full was not being accounted for.

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