btrfs: remove useless 'fail_unlock' label from btrfs_csum_file_blocks()

The label 'fail_unlock' is pointless, all it does is to jump to the label
'out', so just remove it.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>