usb: gadget: u_ether: Fix NULL pointer deref in eth_get_drvinfo

usb: gadget: u_ether: Fix NULL pointer deref in eth_get_drvinfo

Commit ec35c1969650 ("usb: gadget: f_ncm: Fix net_device lifecycle with
device_move") reparents the gadget device to /sys/devices/virtual during
unbind, clearing the gadget pointer. If the userspace tool queries on
the surviving interface during this detached window, this leads to a
NULL pointer dereference.

Unable to handle kernel NULL pointer dereference
Call trace:
 eth_get_drvinfo+0x50/0x90
 ethtool_get_drvinfo+0x5c/0x1f0
 __dev_ethtool+0xaec/0x1fe0
 dev_ethtool+0x134/0x2e0
 dev_ioctl+0x338/0x560

Add a NULL check for dev->gadget in eth_get_drvinfo(). When detached,
skip copying the fw_version and bus_info strings, which is natively
handled by ethtool_get_drvinfo for empty strings.

Suggested-by: Val Packett <val@packett.cool>
Reported-by: Val Packett <val@packett.cool>
Closes: https://lore.kernel.org/linux-usb/10890524-cf83-4a71-b879-93e2b2cc1fcc@packett.cool/
Fixes: ec35c1969650 ("usb: gadget: f_ncm: Fix net_device lifecycle with device_move")
Cc: stable <stable@kernel.org>
Signed-off-by: Kuen-Han Tsai <khtsai@google.com>
Link: https://patch.msgid.link/20260316-eth-null-deref-v1-1-07005f33be85@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>