In station/client mode, the link data needs a bit more
initialization and destruction than just zero-init and
kfree() respectively, implement that.

This required some shuffling of the link data handling
in general, as we should set it up in setup and do the
teardown in teardown, otherwise we're asymmetric in
case of interface type changes.

Also stop using kfree_rcu(), we cannot guarantee that
nothing is scheduling things that live within the link
(e.g. the u.mgd.request_smps_work) until we're sure it
cannot be referenced anymore, therefore synchronize
instead. This isn't very efficient, but we can always
optimize it later.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>