dccp: consistently use dccp_write_space()

DCCP uses dccp_write_space() for sk->sk_write_space method.

Unfortunately a passive connection (as provided by accept())
is using the generic sk_stream_write_space() function.

Lets simply inherit sk->sk_write_space from the parent
instead of forcing the generic one.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>