ethernet/netronome/nfp: Fix a use after free in nfp_bpf_ctrl_msg_rx

In nfp_bpf_ctrl_msg_rx, if
nfp_ccm_get_type(skb) == NFP_CCM_TYPE_BPF_BPF_EVENT is true, the skb
will be freed. But the skb is still used by nfp_ccm_rx(&bpf->ccm, skb).

My patch adds a return when the skb was freed.

Fixes: bcf0cafa

 ("nfp: split out common control message handling code")
Signed-off-by: Lv Yunlong <lyl2019@mail.ustc.edu.cn>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>