net: socket: Release packets in accepted socket in close

If we have received data to the accepted socket, then release
those before removing the accepted socket. This is a rare event
as it requires that we get multiple simultaneous connections
and there is a failure before the socket accept is called by
the application.
For example one such scenario is when HTTP server receives multiple
connection attempts at the same time, and the server poll fails
before socket accept is called. This leads to buffer leak as the
socket close is not called for the accepted socket because the
accepted is not yet created from application point of view.
The solution is to flush the received queue of the accepted socket
before removing the actual accepted socket.

Fixes #84538

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>