bpf: Fix dev pointer dereference from sk_skb

Dan Carpenter reports:

The patch 6acc9b43

: "bpf: Add helper to retrieve socket in BPF"
from Oct 2, 2018, leads to the following Smatch complaint:

    net/core/filter.c:4893 bpf_sk_lookup()
    error: we previously assumed 'skb->dev' could be null (see line 4885)

Fix this issue by checking skb->dev before using it.

Signed-off-by: Joe Stringer <joe@wand.net.nz>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>