uapi: Fix linux/rds.h userspace compilation errors.

Include linux/in6.h for struct in6_addr.

/usr/include/linux/rds.h:156:18: error: field ‘laddr’ has incomplete type
  struct in6_addr laddr;
                  ^~~~~
/usr/include/linux/rds.h:157:18: error: field ‘faddr’ has incomplete type
  struct in6_addr faddr;
                  ^~~~~
/usr/include/linux/rds.h:178:18: error: field ‘laddr’ has incomplete type
  struct in6_addr laddr;
                  ^~~~~
/usr/include/linux/rds.h:179:18: error: field ‘faddr’ has incomplete type
  struct in6_addr faddr;
                  ^~~~~
/usr/include/linux/rds.h:198:18: error: field ‘bound_addr’ has incomplete type
  struct in6_addr bound_addr;
                  ^~~~~~~~~~
/usr/include/linux/rds.h:199:18: error: field ‘connected_addr’ has incomplete type
  struct in6_addr connected_addr;
                  ^~~~~~~~~~~~~~
/usr/include/linux/rds.h:219:18: error: field ‘local_addr’ has incomplete type
  struct in6_addr local_addr;
                  ^~~~~~~~~~
/usr/include/linux/rds.h:221:18: error: field ‘peer_addr’ has incomplete type
  struct in6_addr peer_addr;
                  ^~~~~~~~~
/usr/include/linux/rds.h:245:18: error: field ‘src_addr’ has incomplete type
  struct in6_addr src_addr;
                  ^~~~~~~~
/usr/include/linux/rds.h:246:18: error: field ‘dst_addr’ has incomplete type
  struct in6_addr dst_addr;
                  ^~~~~~~~

Fixes: b7ff8b10 ("rds: Extend RDS API for IPv6 support")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>