sfc: fix null pointer dereference in efx_hard_start_xmit

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: sfc: fix null pointer dereference in efx_hard_start_xmit Trying to get the channel from the tx_queue variable here is wrong because we can only be here if tx_queue is NULL, so we shouldn't dereference it. As the above comment in the code says, this is very unlikely to happen, but it's wrong anyway so let's fix it. I hit this issue because of a different bug that caused tx_queue to be NULL. If that happens, this is the error message that we get here: BUG: unable to handle kernel NULL pointer dereference at 0000000000000020 [...] RIP: 0010:efx_hard_start_xmit+0x153/0x170 [sfc]

References

• https://git.kernel.org/stable/c/b3b41d4d95d3822b2e459ecbc80d030ea6aec5e7
• https://git.kernel.org/stable/c/8547c7bfc0617e7184e4da65b9b96681fcfe9998
• https://git.kernel.org/stable/c/b3b952168ee1f220ba729fa100fd9d5aa752eb03
• https://git.kernel.org/stable/c/0a242eb2913a4aa3d6fbdb86559f27628e9466f3