XFS filesystem recovery from secondary superblocks

[Aaron Goulding]

So an update! I did another search of /dev/md0 for superblock locations
(find all instances of XFSB, find all instances of XAGF that occur 512
bytes after XFSB) and used those as start points to copy the data to
/dev/md1. I took the block count from the superblock, (right at 1TB per AG)
and copied that block of data to the array, properly aligned.

This seems to have worked rather well! xfs_db now has much fewer errors,
and can read SBs and AGFs, and xfs_repair can do something with it.