zygo: btrfs: more debugging from Qu

zygo: btrfs: more debugging from Qu

Mind to apply this snippet?

It's not a poking around fixes, but a debug output for:
- Which block group is being balanced, and when it finishes
- Which subvolume (including reloc tree) is being dropped and when it
finishes

It's going to hit all those bugs again, but with the debug patch, I can
see how impossible things happens.

After some discussion in SUSE internal IRC, it proves one nightmare:
- root->reloc_root assignment is not atomic
- fs_info->reloc_cntl assigment is not atomic

So despite that, would you like to test it on a single core VM or
disabling all other cores of a baremetal to see if it can be reproduced?

Thanks,
Qu