net: ipv6: correctly set the offset to the unknown option

When an unknown option is encountered, an ICMPv6 error message must be
sent in some cases. The message contains a pointer field, which must be
the offset to the unknown option. Currently the offset is computed from
the beginning of the option list, while it should be computed with
respect to the beginning of the IPv6 header.

Record the offset when reading the option type and pass it later to
ipv6_drop_on_unknown_option() to correctly set the pointer field. Also
rename the argument in ipv6_drop_on_unknown_option() to make the
purpose more clear.

Signed-off-by: Florian Vaussard <florian.vaussard@gmail.com>