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

include/net/dsfield.h

@@ -21,7 +21,7 @@ static inline __u8 ipv4_get_dsfield(const struct iphdr *iph)
 
 static inline __u8 ipv6_get_dsfield(const struct ipv6hdr *ipv6h)
 {
-	return ntohs(*(const __be16 *)ipv6h) >> 4;
+	return ntohs(*(__force const __be16 *)ipv6h) >> 4;
 }