Correct a type error in kmem_unback(). Previously, kmem_unback() did not

correctly handle deallocation requests of two or more gigabytes in size.
Eventually, this would lead to a panic elsewhere in the kernel, such as
"vm_radix_insert: key <vm_pindex_t> is already present".

Reported by:	Ilias Marinos
MFC after:	1 week

sys/vm/vm_kern.c

@@ -391,8 +391,7 @@ void
 kmem_unback(vm_object_t object, vm_offset_t addr, vm_size_t size)
 {
 	vm_page_t m;
-	vm_offset_t offset;
-	int i;
+	vm_offset_t i, offset;
 
 	KASSERT(object == kmem_object || object == kernel_object,
 	    ("kmem_unback: only supports kernel objects."));