net: sockets: Allocate FD after receiving connection in accept()

Allocating FD before pulling the new connection from the fifo can lead
to busy looping in certain cases. If the application keeps calling
accept() on a listening socket after failing to allocate new FD for the
incoming connection, it'll start busy looping, as will report POLLIN in
such case (as the new connection is still on the queue), but it'll
consistently fail with ENFILE.

This can be avoided by trying to allocate new file descriptor only after
new connection has been pulled from the fifo. That way, if we fail to
allocate the file descriptor, the incoming connection will be dropped,
which seems correct given we don't have enough resources to service it.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>