[PATCH] Btrfs: cleanup error handling in build_backref_tree

When balance panics it tends to panic in the

BUG_ON(!upper->checked);

test, because it means it couldn't build the backref tree properly.  This is
annoying to users and frankly a recoverable error, nothing in this function is
actually fatal since it is just an in-memory building of the backrefs for a
given bytenr.  So go through and change all the BUG_ON()'s to ASSERT()'s, and
fix the BUG_ON(!upper->checked) thing to just return an error.

This patch also fixes the error handling so it tears down the work we've done
properly.  This code was horribly broken since we always just panic'ed instead
of actually erroring out, so it needed to be completely re-worked.  With this
patch my broken image no longer panics when I mount it.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fb.com>
[PATCH] Btrfs: fix build_backref_tree issue with multiple shared blocks

Marc Merlin sent me a broken fs image months ago where it would blow up in the
upper->checked BUG_ON() in build_backref_tree.  This is because we had a
scenario like this

block a -- level 4 (not shared)
   |
block b -- level 3 (reloc block, shared)
   |
block c -- level 2 (not shared)
   |
block d -- level 1 (shared)
   |
block e -- level 0 (shared)

We go to build a backref tree for block e, we notice block d is shared and add
it to the list of blocks to lookup it's backrefs for.  Now when we loop around
we will check edges for the block, so we will see we looked up block c last
time.  So we lookup block d and then see that the block that points to it is
block c and we can just skip that edge since we've already been up this path.
The problem is because we clear need_check when we see block d (as it is shared)
we never add block b as needing to be checked.  And because block c is in our
path already we bail out before we walk up to block b and add it to the backref
check list.

To fix this we need to reset need_check if we trip over a block that doesn't
need to be checked.  This will make sure that any subsequent blocks in the path
as we're walking up afterwards are added to the list to be processed.  With this
patch I can now mount Marc's fs image and it'll complete the balance without
panicing.  Thanks,

Reported-by: Marc MERLIN <marc@merlins.org>
Signed-off-by: Josef Bacik <jbacik@fb.com>