tcp: Add missing annotation for tcp_child_process()

Sparse reports warning at tcp_child_process()
warning: context imbalance in tcp_child_process() - unexpected unlock
The root cause is the missing annotation at tcp_child_process()

Add the missing __releases(&((child)->sk_lock.slock)) annotation

Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>