x86/mm: avoid undefined behavior in IS_NIL()

x86/mm: avoid undefined behavior in IS_NIL()

Since pointer overflow is undefined behavior in C, some compilers such
as clang optimize away the check !((ptr) + 1) in the macro IS_NIL().

This patch fixes the issue by casting the pointer type to uintptr_t,
the operations of which are well-defined.

Signed-off-by: Xi Wang <xi@mit.edu>
With that, we also need to avoid the overflow in NIL().

Note that either part of the change results in the respective macros to
become unsuitable for use with "void".

Signed-off-by: Jan Beulich <jbeulich@suse.com>