tcp: tcp_mtu_probe() is likely to exit early

Adding a likely() in tcp_mtu_probe() moves its code which used to
be inlined in front of tcp_write_xmit()

We still have a cache line miss to access icsk->icsk_mtup.enabled,
we will probably have to reorganize fields to help data locality.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>