IPv6 NDISC Updates

This series of changes basically clean up NDISC logic, especially on sender side.

We originally do For NS/NA/RS:
1) build temporary ICMPv6 header
2) ndisc_build_skb() with temporary ICMPv6 header and rather criptic arguments.
- Calculate total length and allocate sk_buff
- Build IPv6 header.
- copy ICMPv6 header, additional data and ND options.
- Fill-in ICMPv6 checksum.
Here, structures defined for message format was not used at all, it is difficult to understand what is being sent, and it was not generic.
3) __ndisc_send()
- Allocate temporary dst.
- Send it.

Several issues:
- We could not defer decision if we should/can send some ND option.
- It is hard to see the packet format at a glance.
- ICMPv6 header was built as temporary variable, and then copied to the buffer.
- Some code path for Redirect was not shared.

With these patches, we do:
1) Calculate (or estimate) message length and option length.
2) Allocate skb (via new ndisc_skb_alloc()).
3) Fill-in ICMPv6 message directly using compound literals.
4) Fill-in ICMPv6 checksum
5) Build IPv6 header (including length)
6) Send the packet (via ndisc_send_skb()).
- allocate temporary dst and send it.

- We can defer calculating real length of the packet. For example, we can give up filling some option at when filling in.
- Message is built directly without temporary buffer.
- Structures defined for message format is easier to understand what is being built.
- NS/NA/RS/Redirect share same logic.
- Reduced code/data size:

	   text	   data	    bss	    dec	    hex	filename
	 265407	  14133	   3488	 283028	  45194	old/net/ipv6/ipv6.o
	 264955	  14109	   3488	 282552	  44fb8	new/net/ipv6/ipv6.o

YOSHIFUJI Hideaki (19):
ndisc: Reduce number of arguments for ndisc_fill_addr_option().
ndisc: Move ndisc_opt_addr_space() to include/net/ndisc.h.
ndisc: Use skb_linearize() instead of pskb_may_pull(skb, skb->len).
ndisc: Introduce ndisc_fill_redirect_hdr_option().
ndisc: Introduce ndisc_alloc_skb() helper.
ipv6: Unshare ip6_nd_hdr() and change return type to void.
ndisc: Simplify arguments for ip6_nd_hdr().
ndisc: Set skb->dev and skb->protocol inside ndisc_alloc_skb().
ndisc: Remove dev argument for ndisc_send_skb().
ndisc: Defer building IPv6 header.
ndisc: Reset skb->trasport_headner inside ndisc_alloc_send_skb().
ndisc: Calculate message body length and option length separately.
ndisc: Make ndisc_fill_xxx_option() for sk_buff.
ndisc: Remove icmp6h argument from ndisc_send_skb().
ndisc: Use ndisc_send_skb() for redirect.
ndisc: Fill in ICMPv6 checksum and IPv6 header in ndisc_send_skb().
ndisc: Break down __ndisc_send().
ndisc: Break down ndisc_build_skb() and build message directly.
ndisc: Use compound literals to build redirect message.

 include/net/ipv6.h    |    7 --
 include/net/ndisc.h   |    8 +-
 net/ipv6/ip6_output.c |   33 -----
 net/ipv6/mcast.c      |   29 ++++-
 net/ipv6/ndisc.c      |  331 +++++++++++++++++++++++++------------------------
 5 files changed, 203 insertions(+), 205 deletions(-)

1.7.9.5