net: tcp: Fix context leak during connect

Replaced tcp_out() with tcp_out_ext() when sending
SYN during tcp connect, so we can intercept error value,
beacuse in such case the packet would not be sent at all
and the stack would not trigger any mechanism to flush
this context so it ends up leaking. These scenarios can
arise when the underlaying interface is not properly
configured or is disconnected. The conn is marked to be
closed and is closed before exiting tcp_in().
Since we can now identify this case we can also exit
early in the net_tcp_connect() function and not wait
for any timeout.

Signed-off-by: Daniel Nejezchleb <dnejezchleb@hwg.cz>