nl80211: fix NLA_POLICY_NESTED() arguments

syzbot reported an out-of-bounds read when passing certain
malformed messages into nl80211. The specific place where
this happened isn't interesting, the problem is that nested
policy parsing was referring to the wrong maximum attribute
and thus the policy wasn't long enough.

Fix this by referring to the correct attribute. Since this
is really not necessary, I'll come up with a separate patch
to just pass the policy instead of both, in the common case
we can infer the maxattr from the size of the policy array.

Reported-by:  <syzbot+4157b036c5f4713b1f2f@syzkaller.appspotmail.com>
Cc: stable@vger.kernel.org
Fixes: 9bb7e0f2 ("cfg80211: add peer measurement with FTM initiator API")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>