LinuxKPI: 802.11 plug mbuf leak in error cases

Manually free the mbuf in certain error cases from net80211 to not
leak it.
Note that the differences between ieee80211_input_mimo() and
ieee80211_input_mimo_all(), the former not consuming the mbuf while
the later does, is confusing.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days