xdp: Reset prog in dev_change_xdp_fd when fd is negative

The commit mentioned in the Fixes tag reuses the local prog variable
when looking up an expected_fd. The variable is not reset when fd < 0
causing a detach with the expected_fd set to actually call
dev_xdp_install for the existing program. The end result is that the
detach does not happen.

Fixes: 92234c8f15c8 ("xdp: Support specifying expected existing program when attaching XDP")
Signed-off-by: David Ahern <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Reviewed-by: Jakub Kicinski <[e-mail]>
Reviewed-by: Toke Høiland-Jørgensen <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

net/core/dev.c

···
 	const struct net_device_ops *ops = dev->netdev_ops;
 	enum bpf_netdev_command query;
 	u32 prog_id, expected_id = 0;
-	struct bpf_prog *prog = NULL;
 	bpf_op_t bpf_op, bpf_chk;
+	struct bpf_prog *prog;
 	bool offload;
 	int err;
 
···
 	} else {
 		if (!prog_id)
 			return 0;
+		prog = NULL;
 	}
 
 	err = dev_xdp_install(dev, bpf_op, extack, flags, prog);