iscsi-target: Implement iSCSI target IPv6 address printing.

The iSCSI target configfs code to print out an initiator's IPv6 address
is not fully implemented. This patch uses snprintf() with the "%pI6c"
format string to format the IPv6 address for display purposes.

Signed-off-by: Chris Boot <bootc@bootc.net>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>