btrfs: rename btrfs_item_end_nr to btrfs_item_data_end

The name btrfs_item_end_nr() is a bit of a misnomer, as it's actually
the offset of the end of the data the item points to.  In fact all of
the helpers that we use btrfs_item_end_nr() use data in their name, like
BTRFS_LEAF_DATA_SIZE() and leaf_data().  Rename to btrfs_item_data_end()
to make it clear what this helper is giving us.

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