ipv6: tcp: add rcu locking in tcp_v6_send_synack()

(cherry pick from commit 3e4006f0b86a5ae5eb0e8215f9a9e1db24506977)

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: 45f6fad84cc30 ("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>

Change-Id: I7819b93a8779b79c706796e74aac50d0f811bb70
Fixes: Change-Id: I81775b2269a8263c4e4760b94b9fdd0d5916b31e ("UPSTREAM:
ipv6: add complete rcu protection around np->opt")
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>