Historically only minimal checks were made of a superblock when it was read in as it was assumed that fsck would have been run to correct any errors before attempting to use the filesystem. Recently several bug reports have been submitted reporting kernel panics that can be made by deliberately corrupting filesystem superblocks, see Bug 263979 - [meta] UFS / FFS / GEOM crash (panic) tracking (https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=263979).
This change upgrades the checks that are performed. These additional checks should prevent panics from a corrupted superblock. Although it appears only in one place, the new code will apply to the kernel modules and user applications that read in superblocks.