[MAC80211]: add interface index to key debugfs

Add a new file 'ifindex' to each key's debugfs dir to
allow finding which interface the key was configured on.
This isn't done as a symlink because of possible netdev
name changes.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Michael Wu <flamingice@sourmilk.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>