mac80211: fix minstrel single-rate memory corruption

The minstrel rate controller periodically looks up rate indexes in
a sampling table.  When accessing a specific row and column, minstrel
correctly does a bounds check which, on the surface, appears to handle
the case where mi->n_rates < 2.  However, mi->sample_idx is actually
defined as an unsigned, so the right hand side is taken to be a huge
positive number when negative, and the check will always fail.

Consequently, the RC will overrun the array and cause random memory
corruption when communicating with a peer that has only a single rate.
The max value of mi->sample_idx is around 25 so casting to int should
have no ill effects.

Without the change, uptime is a few minutes under load with an AP
that has a single hard-coded rate, and both the AP and STA could
potentially crash.  With the change, both lasted 12 hours with a
steady load.

Thanks to Ognjen Maric for providing the single-rate clue so I could
reproduce this.

This fixes http://bugzilla.kernel.org/show_bug.cgi?id=12490 on the
regression list (also http://bugzilla.kernel.org/show_bug.cgi?id=13000).

Cc: stable@kernel.org
Reported-by: Sergey S. Kostyliov <rathamahata@gmail.com>
Reported-by: Ognjen Maric <ognjen.maric@gmail.com>
Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/rc80211_minstrel.c

@@ -215,7 +215,7 @@ minstrel_get_next_sample(struct minstrel_sta_info *mi)
 	unsigned int sample_ndx;
 	sample_ndx = SAMPLE_TBL(mi, mi->sample_idx, mi->sample_column);
 	mi->sample_idx++;
-	if (mi->sample_idx > (mi->n_rates - 2)) {
+	if ((int) mi->sample_idx > (mi->n_rates - 2)) {
 		mi->sample_idx = 0;
 		mi->sample_column++;
 		if (mi->sample_column >= SAMPLE_COLUMNS)