fsck_vxfs superblock verification phase

The ``superblock verification phase'' checks to see that the superblock specified a valid vxfs filesystem. It also ensures that all allocation unit (au) headers for the filesystem match the superblock. In normal operation, superblock verification succeeds, except for the following cases:

• The superblock was physically damaged.

• An allocation unit header was physically damaged.

If the superblock is damaged, the superblock can be restored from an auxiliary copy found in the first allocation unit header.

If an allocation unit header is damaged, it can be restored from the superblock.