We have the pattern of

	item = btrfs_item_nr(slot);
	btrfs_set_item_*(leaf, item);

in a bunch of places in our code.  Fix this by adding
btrfs_set_item_*_nr() helpers which will do the appropriate work, and
replace those calls with

	btrfs_set_item_*_nr(leaf, slot);
Signed-off-by: NJosef Bacik <josef@toxicpanda.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>