mac80211: fix use-after-free in defragmentation

Upon receiving the last fragment, all but the first fragment
are freed, but the multicast check for statistics at the end
of the function refers to the current skb (the last fragment)
causing a use-after-free bug.

Since multicast frames cannot be fragmented and we check for
this early in the function, just modify that check to also
do the accounting to fix the issue.

Cc: stable@vger.kernel.org
Reported-by: Yosef Khyal <yosefx.khyal@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>