bpf: Add SO_TXREHASH setsockopt

Add bpf socket option to override rehash behaviour from userspace or from bpf.

Signed-off-by: Akhmat Karakotov <hmukos@yandex-team.ru>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>