ptrace: fixup x86-64 mmu_page_is_user_writable

Fixes #303

Author: klange

base/usr/include/kernel/arch/x86_64/pml.h

@@ -23,4 +23,4 @@ union PML {
 };
 
 #define mmu_page_is_user_readable(p) (p->bits.user)
-#define mmu_page_is_user_writable(p) (p->bits.writable)
+#define mmu_page_is_user_writable(p) (p->bits.user && p->bits.writable)