At update_ref_for_cow() we are calling btrfs_handle_fs_error() if we find
that the extent buffer has an unexpected ref count of zero, however we can
simply use btrfs_abort_transaction(), which achieves the same purposes: to
turn the fs to error state, abort the current transaction and turn the fs
to RO mode as well. Besides that, btrfs_abort_transaction() also prints a
stack trace which makes it more useful.

Also, as this is a very unexpected situation, indicating a serious
corruption/inconsistency, tag the if branch as 'unlikely', set the error
code to -EUCLEAN instead of -EROFS, and log an explicit message.
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>