CVE-2026-98791: net/mlx5e: Fix AF_XDP TX timestamp teardown NULL dereference

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

net/mlx5e: Fix AFXDP TX timestamp teardown NULL dereference

During XSK TX queue teardown, outstanding descriptors are completed without a CQE. If one requested a TX timestamp, the completion path passes that NULL CQE to mlx5exskfilltimestamp(), which dereferences it.

Return zero when no CQE is available, indicating that teardown did not produce a TX timestamp.