KVM: selftests: Cast guest_memfd fd to a signed int when checking for >= 0

KVM: selftests: Cast guest_memfd fd to a signed int when checking for >= 0

When conditionally closing a memory region's guest_memfd file descriptor,
cast the field to a signed it so that negative values are correctly
detected.  Because selftests reuse "struct kvm_userspace_memory_region2"
instead of providing custom storage, they pick up the kernel uAPI's __u32
definition of the file descriptor, not the more common "int" definition,
e.g. that's used for userspace_mem_region.fd.

Fixes: bb2968ad6c33 ("KVM: selftests: Add support for creating private memslots")
Reported-by: Bibo Mao <maobibo@loongson.cn>
Closes: https://lore.kernel.org/all/20260508015013.4108345-1-maobibo@loongson.cn
Reviewed-by: Bibo Mao <maobibo@loongson.cn>
Reviewed-by: Ackerley Tng <ackerleytng@google.com>
Link: https://patch.msgid.link/20260522171535.3525890-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>