修复img文件挂载错误

问题场景：

#mount rootfs.img system
mount: wrong fs type, bad option, bad superblock on /dev/loop0,
       missing codepage or helper program, or other error
       In some cases useful info is found in syslog - try
       dmesg | tail  or so

修复方法：

#fsck.ext4 rootfs.img
e2fsck 1.42.9 (4-Feb-2014)
One or more block group descriptor checksums are invalid.  Fix<y>? yes
Group descriptor 0 checksum is 0x109b, should be 0x744b.  FIXED.
Group descriptor 1 checksum is 0xa1f8, should be 0xc528.  FIXED.
Group descriptor 2 checksum is 0x095b, should be 0x6d8b.  FIXED.
rootfs.fex.iso contains a file system with errors, check forced.
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 3A: Optimizing directories
Pass 4: Checking reference counts
Pass 5: Checking group summary information
rootfs.img: ***** FILE SYSTEM WAS MODIFIED *****
rootfs.img: 825/1224 files (0.7% non-contiguous), 17828/21808 blocks

测试： #mount rootfs.img system
重新挂载正常