When cpu_up() fails, we can discern the most likely cause.

If cpu_present() is false, this means the cpu did not appear
in the MD.  If -ENODEV is the error return value, then
the processor did not boot properly into the kernel.

Pass this information back in the dr-cpu response packet.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>