make IS_ERR_VALUE() complain about non-pointer-sized arguments

Now that the allmodconfig x86-64 build is clean wrt IS_ERR_VALUE() uses
on integers, add a cast to a pointer and back to the argument, so that
any new mis-uses of IS_ERR_VALUE() will cause warnings like

   warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]

so that we don't re-introduce any bogus uses.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>