bpf: allow to propagate df in bpf_skb_set_tunnel_key

Added by 9a628224 ("ip_tunnel: Add dont fragment flag."), allow to
feed df flag into tunneling facilities (currently supported on TX by
vxlan, geneve and gre) as a hint from eBPF's bpf_skb_set_tunnel_key()
helper.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>