CVE-2026-31590: KVM: SEV: Drop WARN on large size for KVM_MEMORY_ENCRYPT_REG_REGION

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

KVM: SEV: Drop WARN on large size for KVMMEMORYENCRYPTREGREGION

Drop the WARN in sevpinmemory() on npages overflowing an int, as the WARN is comically trivially to trigger from userspace, e.g. by doing:

struct kvmencregion range = { .addr = 0, .size = -1ul, };

vmioctl(vm, KVMMEMORYENCRYPTREGREGION, &range);

Note, the checks in sevmemencregisterregion() that presumably exist to verify the incoming address+size are completely worthless, as both "addr" and "size" are u64s and SEV is 64-bit only, i.e. they can't be greater than ULONGMAX. That wart will be cleaned up in the near future.

if (range->addr > ULONGMAX || range->size > ULONGMAX) return -EINVAL;

Opportunistically add a comment to explain why the code calculates the number of pages the "hard" way, e.g. instead of just shifting @ulen.