using a len(net.IP) to check for ipv4 or ipv6 is a bad idea.
And that was exactly done in NetworkOverlaps() function with the
assumption that any ipv4 net.IP will be of 4 bytes. Golang Net package
makes no such assumptions.
This assumption actually broke a particular use-case where the
NetworkOverlaps fails to identify a genuine overlap and that causes
datapath issues.
With this fix, we explicitely check for v4 or v6
Signed-off-by: Madhu Venugopal <madhu@docker.com>