btrfs: Evaluate io_tree in find_lock_delalloc_range()

Simplification.  No point passing the tree variable when it can be
evaluated from inode. The tests now use the io_tree from btrfs_inode as
opposed to creating one.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>