ipv6: Annotate bitwise IPv6 dsfield pointer cast

The sparse commit 6002ded74587 ("add a flag to warn on casts to/from
bitwise pointers") introduced a check for non-direct casts from/to
restricted datatypes (when -Wbitwise-pointer is enabled).

This triggered a warning in ipv6_get_dsfield() because sparse doesn't know
that the buffer already points to some data in the correct bitwise integer
format. This was already fixed in ipv6_change_dsfield() by the __force
attribute and can be fixed here the same way.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>