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: Changli Gao <xiaosuo@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>