kernel: ext4: avoid divide by zero when trying to mount a corrupted file system

The ext4fillflexinfo function in fs/ext4/super.c in the Linux kernel before 2.6.32-git6 allows user-assisted remote attackers to cause a denial of service divide-by-zero error and panic via a malformed ext4 filesystem containing a super block with a large FLEXBG group size aka sloggroupsperflex...