mac80211: backport "cfg80211: limit scan results cache size"

The patch commit states:
"It's possible to make scanning consume almost arbitrary amounts
of memory, e.g. by sending beacon frames with random BSSIDs at
high rates while somebody is scanning.

Limit the number of BSS table entries we're willing to cache to
1000, limiting maximum memory usage to maybe 4-5MB, but lower
in practice - that would be the case for having both full-sized
beacon and probe response frames for each entry; this seems not
possible in practice, so a limit of 1000 entries will likely be
closer to 0.5 MB."

Signed-off-by: Johannes Berg <johannes.berg@intel.com&gt;">

This patch was added in 4.4.36. But because LEDE backports
cfg80211, mac80211 and the wifi drivers separately, it needs
to be added manually for now. It can be dropped later as it
will be part of the next mac80211 refresh.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>