[IPV4]: Do not disable preemption in trie_leaf_remove().

Hello, Just discussed this Patrick...

We have two users of trie_leaf_remove, fn_trie_flush and fn_trie_delete
both are holding RTNL. So there shouldn't be need for this preempt stuff.
This is assumed to a leftover from an older RCU-take.

> Mhh .. I think I just remembered something - me incorrectly suggesting
> to add it there while we were talking about this at OLS :) IIRC the
> idea was to make sure tnode_free (which at that time didn't use
> call_rcu) wouldn't free memory while still in use in a rcu read-side
> critical section. It should have been synchronize_rcu of course,
> but with tnode_free using call_rcu it seems to be completely
> unnecessary. So I guess we can simply remove it.
Signed-off-by: NRobert Olsson <robert.olsson@its.uu.se>
Signed-off-by: NPatrick McHardy <kaber@trash.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/fib_trie.c

@@ -1527,7 +1527,6 @@ static int trie_leaf_remove(struct trie *t, t_key key)
 	t->revision++;
 	t->size--;
 
-	preempt_disable();
 	tp = NODE_PARENT(n);
 	tnode_free((struct tnode *) n);
 
@@ -1537,7 +1536,6 @@ static int trie_leaf_remove(struct trie *t, t_key key)
 		rcu_assign_pointer(t->trie, trie_rebalance(t, tp));
 	} else
 		rcu_assign_pointer(t->trie, NULL);
-	preempt_enable();
 
 	return 1;
 }