extent_io.c/h are huge, encompassing a bunch of different things.  The
extent_io_tree code can live on its own, so separate this out.
Signed-off-by: NJosef Bacik <josef@toxicpanda.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>