CVE-2026-98900: xen/gntalloc: validate grant count before allocation

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.