[NET]: Use NIP6_FMT in kernel.h

There are errors and inconsistency in the display of NIP6 strings.
	ie: net/ipv6/ip6_flowlabel.c

There are errors and inconsistency in the display of NIPQUAD strings too.
	ie: net/netfilter/nf_conntrack_ftp.c

This patch:
	adds NIP6_FMT to kernel.h
	changes all code to use NIP6_FMT
	fixes net/ipv6/ip6_flowlabel.c
	adds NIPQUAD_FMT to kernel.h
	fixes net/netfilter/nf_conntrack_ftp.c
	changes a few uses of "%u.%u.%u.%u" to NIPQUAD_FMT for symmetry to NIP6_FMT

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: David S. Miller <davem@davemloft.net>