[RFC/RFT] mac80211: allow station add/remove to sleep

[Sujith <Sujith.Manoharan@atheros.com>]

Johannes Berg wrote:
> Many drivers would like to sleep during station
> addition and removal, and currently have a high
> complexity there from not being able to.
> 
> This introduces two new callbacks sta_add() and
> sta_remove() that drivers can implement instead
> of using sta_notify() and that can sleep, and
> the new sta_add() callback is also allowed to
> fail.
> 
> The reason we didn't do this previously is that
> the IBSS code wants to insert stations from the
> RX path, which is a tasklet, so cannot sleep.
> This patch will keep the station allocation in
> that path, but moves adding the station to the
> driver out of line. Since the addition can now
> fail, we can have IBSS peer structs the driver
> rejected -- in that case we still talk to the
> station but never tell the driver about it in
> the control.sta pointer. If there will ever be
> a driver that has a low limit on the number of
> stations and that cannot talk to any stations
> that are not known to it, we need to do come up
> with a new strategy of handling larger IBSSs,
> maybe quicker expiry or rejecting peers.

Tested in STA mode with ath9k_htc.
Thanks.

Sujith