nl80211: use the new genetlink pre/post_doit hooks

This makes nl80211 use the new genetlink
pre_doit/post_doit hooks for locking and
checking the interface/wiphy index.

This significantly reduces the code size
and the likelihood of locking errors.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>