btrfs: use the existing reserved items for our first prop for inheritance

We're now reserving an extra items worth of space for property
inheritance.  We only have one property at the moment so this covers us,
but if we add more in the future this will allow us to not get bitten by
the extra space reservation.  If we do add more properties in the future
we should re-visit how we calculate the space reservation needs by the
callers.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
[ refreshed on top of prop/xattr cleanups ]
Signed-off-by: David Sterba <dsterba@suse.com>