net: ipv6: Add helper to get the last extension header

Figure out what is the last extension header in IPv6 packet
and return offset to it. This is needed by IPv6 fragmentation
when fragmentation header is added to the packet.

Change-Id: I925ab806a5de076a425ff354711730d4f4b3c52f
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>