Changelog

• subvolume create: return correct error code when a target already exists
• tree-checker: dump tree block on error (btrfs-convert, ...)
• scrub limit: fix reporting of a limit set while there's none
• fi usage: fix reporting of unallocated data or raid56 profile without root privs due to lack of that information
• convert:
  • align data block group lengths to 64K
  • fix conversion of a large filesystem when there are partial inode items present due to caching
• other:
  • build fixes
  • updated documentation
  • new and updated tests