ext2fs: Add some extra consistency checks for the superblock.

Maliciously formed, or badly corrupted, filesystems can cause kernel
panics. In general, such acts of foot-shooting can only be accomplished
by root, but in a world with VM images that is moving towards automated
mounts it is important to have some form of prevention.

Reported by: Christopher Krah, Thomas Barabosch, and Jan-Niclas Hilgert
of Fraunhofer FKIE.
Incidentaly this should also fix a memory corruption issue reported by
Dr Silvio Cesare of InfoSect.

Huge thanks to all reseachers for making us aware of the issue.

admbug: 872, 891
Reviewed by: fsu
Obtained from: NetBSD (with minor changes)
MFC after: 3 days