RISC-V: Fix handling of empty cpu masks

The cpumask rework slightly changed the behavior of the code.  Fix this
by treating an empty cpumask as meaning all online CPUs.

Extracted from a patch by Atish Patra <atishp@rivosinc.com>.

Reported-by: Jessica Clarke <jrtc27@jrtc27.com>
Fixes: 26fb751c

 ("RISC-V: Do not use cpumask data structure for hartid bitmap")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>