This patch allows GET_INTERFACE dumps to be filtered based on
NL80211_ATTR_WIPHY or NL80211_ATTR_WDEV.  The documentation for
GET_INTERFACE mentions that this is possible:
"Request an interface's configuration; either a dump request on
a %NL80211_ATTR_WIPHY or ..."

However, this behavior has not been implemented until now.

Johannes: rewrite most of the patch:
 * use nl80211_dump_wiphy_parse() to also allow passing an interface
   to be able to dump its siblings
 * fix locking (must hold rtnl around using nl80211_fam.attrbuf)
 * make init self-contained instead of relying on other cb->args
Signed-off-by: Denis Kenzior <denkenz@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>