Fix port leaking after controlling_process(Port, self())

Add case to handle the situation when someone call
{gen_tcp,gen_udp}:controlling_process(Port, self()). Also improve spec
and doc from gen_udp and gen_sctp for controlling_process/2.

To reproduce the issue, open an UDP port:

4> {ok,Port} = gen_udp:open(9000, [binary]).
{ok,#Port<0.587>}
5> gen_udp:controlling_process(Port, self()).
ok

Simulate error:

6> 1=2.
** exception error: no match of right hand side value 2

Here is the leak:

7> inet:i().
Port Module   Recv Sent Owner    Local Address Foreign Address State
Type
581  inet_udp 0    0    <0.31.0> *:cslistener  *:*             BOUND
DGRAM
ok

1 files changed, 4 insertions, 0 deletions

diff --git a/lib/kernel/src/inet.erl b/lib/kernel/src/inet.erl
index 49f64a9236..3356fb55d7 100644
--- a/lib/kernel/src/inet.erl
+++ b/lib/kernel/src/inet.erl
@@ -1244,6 +1244,8 @@ udp_close(S) when is_port(S) ->
 %% Set controlling process for TCP socket.
 tcp_controlling_process(S, NewOwner) when is_port(S), is_pid(NewOwner) ->
     case erlang:port_info(S, connected) of
+	{connected, NewOwner} ->
+	    ok;
 	{connected, Pid} when Pid =/= self() ->
 	    {error, not_owner};
 	undefined ->
@@ -1295,6 +1297,8 @@ tcp_sync_input(S, Owner, Flag) ->
 %% Set controlling process for UDP or SCTP socket.
 udp_controlling_process(S, NewOwner) when is_port(S), is_pid(NewOwner) ->
     case erlang:port_info(S, connected) of
+	{connected, NewOwner} ->
+	    ok;
 	{connected, Pid} when Pid =/= self() ->
 	    {error, not_owner};
 	_ ->