net: Make it easier to parse /proc/net/dev contents.

The contents of /proc/net/dev is annoying to parse, because
it changes whether there is a space after the "ethX:" or not.
It depends upon the size of the "Receive bytes" counter,
if the number is below 7 digits, then there is whitespaces
else if the number is 8 digits or above there is no space
between the ":" and the number.

This patch changes the output to assure there is always a space
between the ":" and the number.  Given that all existing userspace
application already need to handle the whitespaces, I see
no breakage of existing tools.

Signed-off-by: Jesper Dangaard Brouer <hawk@comx.dk>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>