SUNRPC: handle IPv6 PKTINFO when extracting destination address

PKTINFO is needed to scrape the caller's IP address off the socket so
RPC datagram replies are routed correctly.  Fill in missing pieces in
the kernel RPC server's UDP receive path to request IPv6 PKTINFO and
correctly parse the IPv6 cmsg header.

Without this patch, kernel RPC services drop all incoming requests on
UDP on IPv6.

Related commit: 7a37f578



Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Cc: Neil Brown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>