Fix for wrongly matching IPv6 address line problem in GetNicNameByIp

[mchtech]

Modify IPv4 regex pattern from "a.b.c.d" to "a\.b\.c\.d" to avoid matching the line that contains IPv6 address, of which pattern is like 2001:db8::a:b:c:d

We have used ZStack to provide dual-stack access (support by ourselves) and encountered some subtle and magic problems.

If the vrouter use an IPv4 address like A.B.C.D and add an IPv6 address with IPv4-mixed-format like 2001:db8::A:B:C:D, the GetNicNameByIp will return a non-existent interface named "global", leading to trigger another error where this function is called.

The line that contains IPv6 address with style 2001:db8::A:B:C:D will be matched by 'A.B.C.D' , because grep treats it as a regex by default.

inet A.B.C.D/24 brd A.B.C.255 scope global (secondary) ethx
inet6 2001:db8::A:B:C:D/64 scope global

Meanwhile, IPv6 address line that outputed by ip addr do not end with ethx and secondary, so return os[len(os)-1] will return global as interface name and then ......