btrfs: remove BTRFS_LEAF_DATA_OFFSET

This is simply the same thing as btrfs_item_nr_offset(leaf, 0), so
remove this helper and replace it's usage with the above statement.

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