ipv6/exthdrs: accept tlv which includes only padding

In rfc4942 and rfc2460 I cannot find anything which would implicate to
drop packets which have only padding in tlv.

Current behaviour breaks TAHI Test v6LC.1.2.6.

Problem was intruduced in:
9b905fe6 "ipv6/exthdrs: strict Pad1 and PadN check"

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>