mac80211_hwsim: add hashtable with mac address keys for faster lookup

This patch adds a rhastable for mac address lookup of hwsim radios. This
especially improve the speed on reception of a netlink message with a new
frame. Although redundant, we keep holding a normal list for all radios,
since the rhashtable_walk interface adds a lot of overhead for iterating
over all radios and the doc of rhashtable recommend a redundant structure
for stable walks in such situations.

Since rhashtable is rcu protected we do not need a lock for delivering
frames and thus improving this scenario.

Signed-off-by: Benjamin Beichler <benjamin.beichler@uni-rostock.de>
[remove hash function, it would cause alignment issues]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>