ipv6: add ipv6_addr_prefix_copy

This patch adds a static inline function ipv6_addr_prefix_copy which
copies a ipv6 address prefix(argument pfx) into the ipv6 address prefix.
The prefix len is given by plen as bits. This function mainly based on
ipv6_addr_prefix which copies one address prefix from address into a new
ipv6 address destination and zero all other address bits.

The difference is that ipv6_addr_prefix_copy don't get a prefix from an
ipv6 address, it sets a prefix to an ipv6 address with keeping other
address bits. The use case is for context based address compression
inside 6LoWPAN IPHC header which keeping ipv6 prefixes inside a context
table to lookup address-bits without sending them.

Cc: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Cc: James Morris <jmorris@namei.org>
Cc: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
Cc: Patrick McHardy <kaber@trash.net>
Acked-by: Łukasz Duda <lukasz.duda@nordicsemi.no>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Acked-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Acked-by: David S. Miller <davem@davemloft.net>
Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

include/net/ipv6.h

@@ -401,6 +401,21 @@ static inline void ipv6_addr_prefix(struct in6_addr *pfx,
 		pfx->s6_addr[o] = addr->s6_addr[o] & (0xff00 >> b);
 }
 
+static inline void ipv6_addr_prefix_copy(struct in6_addr *addr,
+					 const struct in6_addr *pfx,
+					 int plen)
+{
+	/* caller must guarantee 0 <= plen <= 128 */
+	int o = plen >> 3,
+	    b = plen & 0x7;
+
+	memcpy(addr->s6_addr, pfx, o);
+	if (b != 0) {
+		addr->s6_addr[o] &= ~(0xff00 >> b);
+		addr->s6_addr[o] |= (pfx->s6_addr[o] & (0xff00 >> b));
+	}
+}
+
 static inline void __ipv6_addr_set_half(__be32 *addr,
 					__be32 wh, __be32 wl)
 {