usb: gadget: u_ether: Re-attach netif device to mirror detachment

In 6.7-rc1, there was a netif_device_detach call added to the
gether_disconnect function. This clears the __LINK_STATE_PRESENT bit of
the netif device and suppresses pings (ICMP messages) and TCP connection
requests from the connected host. If userspace temporarily disconnects
the gadget, such as by temporarily removing configuration in the gadget
configfs interface, network activity should continue to be processed
when the gadget is re-connected. Mirror the netif_device_detach call
with a netif_device_attach call in gether_connect to fix re-connecting
gadgets.

Link: https://gitlab.com/postmarketOS/pmaports/-/tree/6002e51b7090aeeb42947e0ca7ec22278d7227d0/main/postmarketos-base-ui/rootfs-usr-lib-NetworkManager-dispatcher.d-50-tethering.sh
Cc: stable <stable@kernel.org>
Fixes: f49449fb

 ("usb: gadget: u_ether: Replace netif_stop_queue with netif_device_detach")
Signed-off-by: Richard Acayan <mailingradian@gmail.com>
Tested-by: Luca Weiss <luca@z3ntu.xyz>
Tested-by: Duje Mihanović <duje.mihanovic@skole.hr>
Link: https://lore.kernel.org/r/20231218164532.411125-2-mailingradian@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>