xen/privcmd: fix double free via VMA splitting

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

xen/privcmd: fix double free via VMA splitting

privcmd_vm_ops defines .close (privcmd_close), but neither .may_split

nor .open. When userspace does a partial munmap() on a privcmd mapping,

the kernel splits the VMA via __split_vma(). Since may_split is NULL,

the split is allowed. vm_area_dup() copies vm_private_data (a pages

array allocated in alloc_empty_pages()) into the new VMA without any

fixup, because there is no .open callback.

Both VMAs now point to the same pages array. When the unmapped portion

is closed, privcmd_close() calls:

- xen_unmap_domain_gfn_range()

- xen_free_unpopulated_pages()

- kvfree(pages)

The surviving VMA still holds the dangling pointer. When it is later

destroyed, the same sequence runs again, which leads to a double free.

Fix this issue by adding a .may_split callback denying the VMA split.

This is XSA-487 / CVE-2026-31787