CVE-2026-99287: xen-netback: Zero out sring pointers on unmap

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

xen-netback: Zero out sring pointers on unmap

If xenvifmapfrontenddatarings() fails when a ring is already mapped, it unmaps them via xenvifunmapfrontenddatarings() which keeps the ring pointers unchanged. xenvifconnectdata()'s cleanup path calls xenvifdisconnectqueue() which in turn calls xenvifunmapfrontenddatarings() again, which will attempt to unmap the pointers a second time.

Zero out the ring pointers after they are unmapped to avoid this issue.

This is XSA-532