xdp: linearize skb in netif_receive_generic_xdp()

In netif_receive_generic_xdp(), it is necessary to linearize all
nonlinear skb. However, in current implementation, skb with
troom <= 0 are not linearized. This patch fixes this by calling
skb_linearize() for all nonlinear skb.

Fixes: de8f3a83 ("bpf: add meta pointer for direct access")
Signed-off-by: Song Liu <songliubraving@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>