dccp: Add missing annotation for dccp_child_process()

Sparse reports a warning at dccp_child_process()
warning: context imbalance in dccp_child_process() - unexpected unlock
The root cause is the missing annotation at dccp_child_process()
Add the missing __releases(child) annotation

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