uml: fix kernel stack size on x86_64

Force KERNEL_STACK_ORDER to be at least 1 on UML/x86_64, to avoid overflows.
Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/um/Kconfig

@@ -278,6 +278,7 @@ config HIGHMEM
 config KERNEL_STACK_ORDER
 	int "Kernel stack size order"
 	default 1 if 64BIT
+	range 1 10 if 64BIT
 	default 0 if !64BIT
 	help
 	This option determines the size of UML kernel stacks.  They will