btrfs: convert BUG_ON()'s in select_reloc_root() to proper errors

We have several BUG_ON()'s in select_reloc_root() that can be tripped if
there is an extent tree corruption.  Convert these to ASSERT()'s, because
if we hit it during testing it really is bad, or could indicate a
problem with the backref walking code.

However if users hit these problems it generally indicates corruption,
I've hit a few machines in the fleet that trip over these with clearly
corrupted extent trees, so be nice and print out an error message and
return an error instead of bringing the whole box down.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>