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

In the Linux kernel, the following vulnerability has been resolved:

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.