ipv6: support IPV6_PMTU_INTERFACE on sockets

IPV6_PMTU_INTERFACE is the same as IPV6_PMTU_PROBE for ipv6. Add it
nontheless for symmetry with IPv4 sockets. Also drop incoming MTU
information if this mode is enabled.

The additional bit in ipv6_pinfo just eats in the padding behind the
bitfield. There are no changes to the layout of the struct at all.

Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/ipv6.h

diff --git a/include/linux/ipv6.h b/include/linux/ipv6.h
index 3fde066..7e1ded0 100644
--- a/include/linux/ipv6.h
+++ b/include/linux/ipv6.h
@@ -191,7 +191,7 @@
 	/* sockopt flags */
 	__u16			recverr:1,
 	                        sndflow:1,
-				pmtudisc:2,
+				pmtudisc:3,
 				ipv6only:1,
 				srcprefs:3,	/* 001: prefer temporary address
 						 * 010: prefer public address