mac80211: debugfs: calculate free buffer size correctly

mac80211: debugfs: calculate free buffer size correctly

In breaking patch buf memory moved from stack to heap and sizeof(buf)
change from size of actual memory to size of the pointer to the heap.

Fix this by holding a separated variable for allocate size.

Fixes: 01f84f0ed3b4 ("mac80211: reduce stack usage in debugfs")
Signed-off-by: Mordechay Goodstein <mordechay.goodstein@intel.com>
Link: https://lore.kernel.org/r/20211021163035.b9ae48c06e27.I6a6ed197110eae28cf4f6e38ce36828a7c136337@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>