af_packet: fix freeing pg_vec twice on error path

It is introduced in:
        commit 0e3125c7
        Author: Neil Horman <nhorman@tuxdriver.com>
        Date:   Tue Nov 16 10:26:47 2010 -0800

        packet: Enhance AF_PACKET implementation to not require high order contiguous memory allocation (v4)
Signed-off-by: NChangli Gao <xiaosuo@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/packet/af_packet.c

@@ -2409,7 +2409,6 @@ static struct pgv *alloc_pg_vec(struct tpacket_req *req, int order)
 
 out_free_pgvec:
 	free_pg_vec(pg_vec, order, block_nr);
-	kfree(pg_vec);
 	pg_vec = NULL;
 	goto out;
 }