BACKPORT: ipv6: tcp: add rcu locking in tcp_v6_send_synack()

(cherry pick from commit 3e4006f0)

When first SYNACK is sent, we already hold rcu_read_lock(), but this
is not true if a SYNACK is retransmitted, as a timer (soft) interrupt
does not hold rcu_read_lock()

Fixes: 45f6fad8 ("ipv6: add complete rcu protection around np->opt")
Reported-by: Dave Jones <davej@codemonkey.org.uk>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Fixes: Change-Id: I9654ba44a4a710a08f8b23bfd8205d205a95607c ("BACKPORT:
ipv6: add complete rcu protection around np->opt")
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>