net/ipv6/exthdrs.c: Strict PadN option checking

Added strict checking of PadN, as PadN can be used to increase header
size and thus push the protocol header into the 2nd fragment.

PadN is used to align the options within the Hop-by-Hop or
Destination Options header to 64-bit boundaries. The maximum valid
size is thus 7 bytes.
RFC 4942 recommends to actively check the "payload" itself and
ensure that it contains only zeroes.

See also RFC 4942 section 2.1.9.5.

Signed-off-by: Eldad Zack <eldad@fogrefinery.com>
Signed-off-by: David S. Miller <davem@davemloft.net>