This uses two things that might be Linux specific,
__builtin_constant_p (might be gcc)
and
BUILD_BUG_ON. (maybe other dm's can have something similiar).

This saves 4k in the bw calcs code.
Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>