CVE-2026-74678: net: usb: ax88179_178a: fix skb leak in ax88179_tx_fixup()

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

net: usb: ax88179178a: fix skb leak in ax88179txfixup()

When the interface has NETIFFSG enabled and skblinearize() fails in ax88179txfixup(), the function returns NULL without freeing the skb.

usbnetstartxmit() treats a NULL return from txfixup() as a drop (info->flags does not set FLAGMULTIPACKET for this driver), jumping to the "drop" label where it does if (skb) devkfreeskbany(skb). Because txfixup() returned NULL, the local skb variable in usbnetstartxmit() is NULL, so the original skb is never freed — a memory leak on every TX frame whose linearization fails (i.e. under memory pressure).

Free the skb before returning, matching the error handling already used for the pskbexpandhead() failure path in the same function.