Merge branch 'bpf_setsockopt-SO_BINDTODEVICE'

Ferenc Fejes says:

====================
This option makes it possible to programatically bind sockets
to netdevices. With the help of this option sockets
of VRF unaware applications could be distributed between
multiple VRFs with an eBPF program. This lets the applications
benefit from multiple possible routes.

v2:
- splitting up the patch to three parts
- lock_sk parameter for optional locking in sock_bindtoindex - Stanislav Fomichev
- testing the SO_BINDTODEVICE option - Andrii Nakryiko
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>