target/mips: Use correct MMU index in get_pte()

When refactoring page_table_walk_refill() in commit 4e999bf419
we missed the indirect call to cpu_mmu_index() in get_pte():

  page_table_walk_refill()
  -> get_pte()
     -> cpu_ld[lq]_code()
        -> cpu_mmu_index()

Since we don't mask anymore the modes in hflags, cpu_mmu_index()
can return UM or SM, while we only expect KM or ERL.

Fix by propagating ptw_mmu_idx to get_pte(), and use the
cpu_ld/st_code_mmu() API with the correct MemOpIdx.

Reported-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reported-by: Waldemar Brodkorb <wbx@uclibc-ng.org>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2470
Fixes: 4e999bf419 ("target/mips: Pass ptw_mmu_idx down from mips_cpu_tlb_fill")
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20240814090452.2591-3-philmd@linaro.org>