rc80211-pid: fix last_sample initialization

Fix last_sample initialization. kzalloc'ing the per-STA data wasn't enough,
as jiffies can assume negative values as well. This fixes a bug which
prevented correct PID operation for a while after booting.

Thanks to Michael Buesch for reporting this.
Reported-and-tested-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: Stefano Brivio <stefano.brivio@polimi.it>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/rc80211_pid_algo.c

@@ -496,6 +496,8 @@ static void *rate_control_pid_alloc_sta(void *priv, gfp_t gfp)
 	if (spinfo == NULL)
 		return NULL;
 
+	spinfo->last_sample = jiffies;
+
 #ifdef CONFIG_MAC80211_DEBUGFS
 	spin_lock_init(&spinfo->events.lock);
 	init_waitqueue_head(&spinfo->events.waitqueue);