netlink: add helper for dissecting an array of attributes

netlink: add helper for dissecting an array of attributes

From https://wiki.linuxfoundation.org/networking/generic_netlink_howto
"Arrays can be represented by using a single nested attribute as a
container with several of the same attribute type inside each
representing a spot in the array."

This patch adds a helper function to dissect such a type (it will be
used by Generic Netlink for CTRL_ATTR_OPS and CTRL_ATTR_MCAST_GROUPS).

Change-Id: I60537712f7fb0f9d4fb8850c6c8eef590635f49f
Reviewed-on: https://code.wireshark.org/review/20903
Petri-Dish: Jaap Keuter <jaap.keuter@xs4all.nl>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>