SUNRPC: Display compressed (shorthand) IPv6 presentation addresses

Recent changes to snprintf() introduced the %pI6c formatter, which can
display an IPv6 address with standard shorthanding.  Using a
shorthanded address can save us a few bytes of memory for each stored
presentation address, or a few bytes on the wire when sending these in
a universal address.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>