bpf: Fix unused-var without NETDEVICES

A recent commit added new variables only used if CONFIG_NETDEVICES is
set. A simple fix would be to only declare these variables if the same
condition is valid but Alexei suggested an even simpler solution:

    since CONFIG_NETDEVICES doesn't change anything in .h I think the
    best is to remove #ifdef CONFIG_NETDEVICES from net/core/filter.c
    and rely on sock_bindtoindex() returning ENOPROTOOPT in the extreme
    case of oddly configured kernels.

Fixes: 70c58997 ("bpf: Allow SO_BINDTODEVICE opt in bpf_setsockopt")
Suggested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200603190347.2310320-1-matthieu.baerts@tessares.net