bug(mount): 无法正常挂载FAT跟EXT4镜像文件

[oeasy1412]

重现步骤

sudo bash tools/make_fs_image.sh 

# make run-nographic 正常进系统
/ # ls /dev # 能看到块设备 vda1 vdb vdc1
block  console  kvm   ptmx  tty    vda1  vdc1
char   hvc0     null  pts   ttyS0  vdb   zero

# 但是测例失败
/ # test_mount_fat
Mount failed with error code: 0
mount costing time: 4000000 ns
Mount failed with error code: 22

thread 'main' (12) panicked at src/bin/test_mount_fat.rs:45:5:
assertion `left == right` failed: Umount fat failed
  left: -1
 right: 0
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
/ # test_mount_ext4
Mount failed with error code: 0
mount costing time: 4000000 ns
Mount failed with error code: 22

thread 'main' (12) panicked at src/bin/test_mount_ext4.rs:44:5:
assertion `left == right` failed: Umount ext4 failed
  left: -1
 right: 0
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace