mac80211: Limit TID buffering during BA session setup/teardown

mac80211: Limit TID buffering during BA session setup/teardown

While setting up or tearing down a BA session mac80211 is buffering
pending frames for the according TID. However, there's currently no
limit on how many frames are buffered possibly leading to an out-of-
memory situation. This can happen on systems with little memory when
the CPU is fully loaded since the BA session work is executed in
process context while frames can still come via softirq.

Apply a limitation to the TIDs pending queue to avoid consuming
too much memory in this situation.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>