mac80211: Fix debugfs union misuse and pointer corruption

debugfs union in struct ieee80211_sub_if_data is misused by including a
common default_key dentry as a union member. This ends occupying the same
memory area with the first dentry in other union members (structures;
usually drop_unencrypted). Consequently, debugfs operations on
default_key symlinks and drop_unencrypted entry are using the same
dentry pointer even though they are supposed to be separate ones. This
can lead to removing entries incorrectly or potentially leaving
something behind since one of the dentry pointers gets lost.

Fix this by moving the default_key dentry to a new struct
(common_debugfs) that contains dentries (more to be added in future)
that are shared by all vif types. The debugfs union must only be used
for vif type-specific entries to avoid this type of pointer corruption.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>