UPSTREAM: net/tcp-fastopen: refactor cookie check logic

Refactor the cookie check logic in tcp_send_syn_data() into a function.
This function will be called else where in later changes.

Bug: 63449462
Test: Tests in https://android-review.googlesource.com/535357/

 pass
Change-Id: I14b0fadd8f97569f773a2e2f15f0b4e8dca48402
Signed-off-by: Wei Wang <weiwan@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry-picked from commit 065263f40f0972d5f1cd294bb0242bd5aa5f06b2)