tcp: add a helper for setting EOR on tail skb

tcp: add a helper for setting EOR on tail skb

TLS (and hopefully soon PSP will) use EOR to prevent skbs
with different decrypted state from getting merged, without
adding new tests to the skb handling. In both cases once
the connection switches to an "encrypted" state, all subsequent
skbs will be encrypted, so a single "EOR fence" is sufficient
to prevent mixing.

Add a helper for setting the EOR bit, to make this arrangement
more explicit.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>