fix: remove stale NB-path comment, improve busybox applet invocation

• virtio_blk.rs: drop "当前有bug，不能走nb路径" comment (NB path already fixed)
• initproc.rs: simple commands invoke busybox applet directly instead of going through sh -c, fixing "command not found" (exit 127) for touch/cat and other non-builtin applets. Only shell metacharacters (>, |, [, ", etc.) now route through sh -c. Busybox test pass rate: 35/66 → 55/66.