btrfs: replace BUG_ON() with error handling at update_ref_for_cow()

In the Linux kernel, the following vulnerability has been resolved:

btrfs: replace BUG_ON() with error handling at update_ref_for_cow()

Instead of a BUG_ON() just return an error, log an error message and

abort the transaction in case we find an extent buffer belonging to the

relocation tree that doesn't have the full backref flag set. This is

unexpected and should never happen (save for bugs or a potential bad

memory).