net: Build IPv6 into kernel by default

This patch makes the default to build IPv6 into the kernel. IPv6
now has significant traction and any remaining vestiges of IPv6
not being provided parity with IPv4 should be swept away. IPv6 is now
core to the Internet and kernel.

Points on IPv6 adoption:

- Per Google statistics, IPv6 usage has reached 7% on the Internet
  and continues to exhibit an exponential growth rate
  https://www.google.com/intl/en/ipv6/statistics.html


- Just a few days ago ARIN officially depleted its IPv4 pool
- IPv6 only data centers are being successfully built
  (e.g. at Facebook)

This patch changes the IPv6 Kconfig for IPV6. Default for CONFIG_IPV6
is set to "y" and the text has been updated to reflect the maturity of
IPv6.

Impact:

Under some circumstances building modules in to kernel might have a
performance advantage. In my testing, I did notice a very slight
improvement.

This will obviously increase the size of the kernel image. In my
configuration I see:

IPv6 as module:

   text    data     bss     dec     hex filename
9703666 1899288  933888 12536842         bf4c0a vmlinux

IPv6 built into kernel

  text     data     bss     dec     hex filename
9436490 1879600  913408 12229498         ba9b7a vmlinux

Which increases text size by ~270K (2.8% increase in size for me). If
image size is an issue, presumably for a device which does not do IP
networking (IMO we should be discouraging IPv4-only devices), IPV6 can
be disabled or still built as a module.

Acked-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>