ipv6: Annotate ipv6_addr_is_* bitwise pointer casts

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 the 64 bit optimized ipv6_addr_is_*() functions
because sparse doesn't know that the buffer already points to some data in
the correct bitwise integer format. But these were correct and can
therefore be marked with __force to signalize sparse an intended cast to a
specific bitwise type.

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