net: tun: split run_ebpf_filter() and pskb_trim() into different "if statement"

No functional change.

Just to split the if statement into different conditions to use
kfree_skb_reason() to trace the reason later.

Cc: Joao Martins <[e-mail]>
Cc: Joe Jin <[e-mail]>
Signed-off-by: Dongli Zhang <[e-mail]>
Reviewed-by: David Ahern <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/tun.c

···
 		goto drop;
 
 	len = run_ebpf_filter(tun, skb, len);
-	if (len == 0 || pskb_trim(skb, len))
+	if (len == 0)
+		goto drop;
+
+	if (pskb_trim(skb, len))
 		goto drop;
 
 	if (unlikely(skb_orphan_frags_rx(skb, GFP_ATOMIC)))