ethtool: provide string sets with STRSET_GET request

Requests a contents of one or more string sets, i.e. indexed arrays of
strings; this information is provided by ETHTOOL_GSSET_INFO and
ETHTOOL_GSTRINGS commands of ioctl interface. Unlike ioctl interface, all
information can be retrieved with one request and mulitple string sets can
be requested at once.

There are three types of requests:

  - no NLM_F_DUMP, no device: get "global" stringsets
  - no NLM_F_DUMP, with device: get string sets related to the device
  - NLM_F_DUMP, no device: get device related string sets for all devices

Client can request either all string sets of given type (global or device
related) or only specific sets. With ETHTOOL_A_STRSET_COUNTS flag set, only
set sizes (numbers of strings) are returned.

Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>