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

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

[ Upstream commit b56329a782314fde5b61058e2a25097af7ccb675 ]

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).

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>