UBUNTU-CVE-2026-98900: Medium severity ubuntu/linux vulnerability

In the Linux kernel, the following vulnerability has been resolved: xen/gntalloc: validate grant count before allocation gntallocioctlalloc() allocates the grant-id array before checking whether the requested count fits within the global grant limit. Counts above that limit cannot succeed, so reject them before the user-controlled allocation reaches kcalloc(). Use a subtraction-based check while holding grefmutex so adding the requested count cannot wrap. Also cast the count before advancing the per-file index so the page-size multiplication is performed in 64-bit arithmetic.