btrfs: set DIRTY and WRITEBACK tags on the buffer_tree

In preparation for changing how we do writeout of extent buffers, start
tagging the extent buffer xarray with DIRTY and WRITEBACK to make it
easier to find extent buffers that are in either state.

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