refcount: Annotated intentional signed integer wrap-around

Mark the various refcount_t functions with __signed_wrap, as we depend
on the wrapping behavior to detect the overflow and perform saturation.
Silences warnings seen with the LKDTM REFCOUNT_* tests:

  UBSAN: signed-integer-overflow in ../include/linux/refcount.h:189:11
  2147483647 + 1 cannot be represented in type 'int'

Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://lore.kernel.org/r/20240221051634.work.287-kees@kernel.org


Signed-off-by: Kees Cook <keescook@chromium.org>