btrfs: always lock the block before calling btrfs_clean_tree_block

We want to clean up the dirty handling for extent buffers so it's a
little more consistent, so skip the check for generation == transid and
simply always lock the extent buffer before calling btrfs_clean_tree_block.

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