riscv: mm: Fix prototype to avoid discarding const

riscv: mm: Fix prototype to avoid discarding const

__flush_tlb_range() does not modify the provided cpumask, so its cmask
parameter can be pointer-to-const. This avoids the unsafe cast of
cpu_online_mask.

Fixes: 54d7431af73e ("riscv: Add support for BATCHED_UNMAP_TLB_FLUSH")
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20240301201837.2826172-1-samuel.holland@sifive.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>