[PATCH 0/5][v2] Relocation and backref resolution fixes

[Josef Bacik <josef@toxicpanda.com>]

v1->v2:
- reworded the first patch.
- Added "btrfs: restart snapshot delete if we have to end the transaction".
  Zygo still was able to hit the backref walking+snapshot delete deadlock
  because the original fix is still a little racey.  We can already restart
  snapshot deletes, just drop our path if we have to end the transaction so
  we're not holding locks across trans handles.

===================== Original email =====================================
These are standalone fixes that came out of my debugging Zygo's problems.  The
first two address a problem with how we handle restarting relocation.
Previously this rarely happened, because if it had people would have complained.
The restart logic was broken in a few subtle ways, and these two patches address
those issues.

The third patch just boggles my mind.  We were recording reloc roots based on
their current bytenr.  This worked fine if we never restarted, but broke if we
had to lookup a ref to a reloc root that we found on the tree.  This is because
that would point at the commit root of the reloc root, but if we had modified
the reloc root we'd no longer be able to find it.

And finally the last one was a weird deadlock that Zygo's insane test rig found,
as he runs the dedup thing while balancing and deleting snapshots, which made
this thing fall out.  Thanks,

Josef