vsprintf: Update %pI6c to not compress a single 0

RFC 5952 (http://tools.ietf.org/html/rfc5952

) mandates that 2 or more
consecutive 0's are required before using :: compression.

Update ip6_compressed_string to match the RFC and update the http
reference as well.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>