NFS: Display compressed (shorthand) IPv6 in /proc/mounts

Recent changes to snprintf() introduced the %pI6c formatter, which can
display an IPv6 address with standard shorthanding.  Use this new
formatter when displaying IPv6 server addresses in /proc/mounts.

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