crimson/os: use a constexpr function for sanity test

crimson/os: use a constexpr function for sanity test

since match_stage_t is an alias of uint_8. it is an unsigned type, so an
instance of this type should be always greater or equal to 0.

this silences warning like:

stage_types.h:37:50: warning: comparison is always true due to limited range of data type [-Wtype-limits]
   37 |     static_assert(STAGE >= STAGE_BOTTOM && STAGE <= STAGE_TOP);
      |                                            ~~~~~~^~~~~~~~~~~~

also, GCC complains if it is able to assure that the compare always
returns true. so a function helps to silence it.

this warning is meaningless per-se, as we are using static_assert() to
perform sanity test at *compile-time*.

Signed-off-by: Kefu Chai <kchai@redhat.com>