netem: dont call vfree() under spinlock and BH disabled

commit 6373a9a2 (netem: use vmalloc for distribution table) added a
regression, since vfree() is called while holding a spinlock and BH
being disabled.

Fix this by doing the pointers swap in critical section, and freeing
after spinlock release.

Also add __GFP_NOWARN to the kmalloc() try, since we fallback to
vmalloc().
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_netem.c

@@ -488,7 +488,7 @@ static int get_dist_table(struct Qdisc *sch, const struct nlattr *attr)
 		return -EINVAL;
 
 	s = sizeof(struct disttable) + n * sizeof(s16);
-	d = kmalloc(s, GFP_KERNEL);
+	d = kmalloc(s, GFP_KERNEL | __GFP_NOWARN);
 	if (!d)
 		d = vmalloc(s);
 	if (!d)
@@ -501,9 +501,10 @@ static int get_dist_table(struct Qdisc *sch, const struct nlattr *attr)
 	root_lock = qdisc_root_sleeping_lock(sch);
 
 	spin_lock_bh(root_lock);
-	dist_free(q->delay_dist);
-	q->delay_dist = d;
+	swap(q->delay_dist, d);
 	spin_unlock_bh(root_lock);
+
+	dist_free(d);
 	return 0;
 }