Black Hole Mass Scaling Relations for Spiral Galaxies. II. $M_{\rm
  BH}$-$M_{\rm *,tot}$ and $M_{\rm BH}$-$M_{\rm *,disk}$
  Black hole mass ($M_{BH}$) scaling relations are typically derived using the
properties of a galaxy's bulge and samples dominated by (high-mass) early-type
galaxies. Studying late-type galaxies should provide greater insight into the
mutual growth of black holes and galaxies in more gas-rich environments. We
have used 40 spiral galaxies to establish how $M_{BH}$ scales with both the
total stellar mass ($M_{*,tot}$) and the disk's stellar mass, having measured
the spheroid (bulge) stellar mass ($M_{*,sph}$) and presented the
$M_{BH}$-$M_{*,sph}$ relation in Paper I. The relation involving $M_{*,tot}$
may be beneficial for estimating $M_{BH}$ either from pipeline data or at
higher redshift, conditions that are not ideal for the accurate isolation of
the bulge. A symmetric Bayesian analysis finds
$\log\left(M_{BH}/M_{\odot}\right)=\left(3.05_{-0.49}^{+0.57}\right)\log\left\{M_{*,tot}/[\upsilon(6.37\times10^{10}\,M_{\odot})]\right\}+(7.25_{-0.14}^{+0.13})$.
The scatter from the regression of $M_{BH}$ on $M_{*,tot}$ is 0.66 dex; compare
0.56 dex for $M_{BH}$ on $M_{*,sph}$ and $0.57$ dex for $M_{BH}$ on $\sigma_*$.
The slope is $>2$ times that obtained using core-S\'ersic early-type galaxies,
echoing a similar result involving $M_{*,sph}$, and supporting a varied growth
mechanism among different morphological types. This steeper relation has
consequences for galaxy/black hole formation theories, simulations, and
predicting black hole masses. We caution that (i) an $M_{BH}$-$M_{*,tot}$
relation built from a mixture of early- and late-type galaxies will find an
arbitrary slope of approximately 1-3, with no physical meaning beyond one's
sample selection, and (ii) evolutionary studies of the $M_{BH}$-$M_{*,tot}$
relation need to be mindful of the galaxy types included at each epoch. We
additionally update the $M_{*,tot}$-($\textit{face-on}$ spiral arm pitch angle)
relation.
