net: core: Fix IPv6 extra header options handling

If the packet contains IPv6 header + Extra header + ICMPv6 headers,
last header does not contain 'next header and length' fields. So
while parsing header we should first check next header and continue
parsing. Otherwise we parse 2 bytes extra and set invalid ext hdr len
and handover to other layers. So everything goes wrong after that.

Also simplified next header parsing to minimal.

Change-Id: I5d8235c5a57c8b77adf6a07eb8eb74350a3e4dc6
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>