tcp: enable sockets to use MSG_FASTOPEN by default

tcp: enable sockets to use MSG_FASTOPEN by default

Applications have started to use Fast Open (e.g., Chrome browser has
such an optional flag) and the feature has gone through several
generations of kernels since 3.7 with many real network tests. It's
time to enable this flag by default for applications to test more
conveniently and extensively.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Cheoleun Moon <chleun.moon@samsung.com>
[jh80.chung: cherry picked from commit 0d41cca490c2 from mainline]

Change-Id: I9dc9ce6e3e8de16bbefd5fd8c79ce9e4b39c4e81