Linux kernel headers violate RFC2553

[Felix von Leitner <leitner@fefe.de>]

glibc works around this, but the diet libc uses the kernel headers and
thus exports the wrong API to user land.

Here is what RFC2553 mandates:

   struct ipv6_mreq {
       struct in6_addr ipv6mr_multiaddr; /* IPv6 multicast addr */
       unsigned int    ipv6mr_interface; /* interface index */
   };

...and here is what include/linux/in6.h declares:

  struct ipv6_mreq {
	  /* IPv6 multicast address of group */
	  struct in6_addr ipv6mr_multiaddr;

	  /* local IPv6 address of interface */
	  int             ipv6mr_ifindex;
  };

Note the ipv6mr_ifindex instead of the correct ipv6mr_interface.

This wrong name is only used twice in net/ipv6/ipv6_sockglue.c, so it should be
trivial to fix.

Felix