tcp: RTT metrics scaling

Some of the metrics (RTT, RTTVAR and RTAX_RTO_MIN) are stored in
kernel units (jiffies) and this leaks out through the netlink API to
user space where the units for jiffies are unknown.

This patches changes the kernel to convert to/from milliseconds. This
changes the ABI, but milliseconds seemed like the most natural unit
for these parameters.  Values available via syscall in
/proc/net/rt_cache and netlink will be in milliseconds.

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