MFC: r254005

- Fix a bug in the MSI allocation logic so an MSI is also employed if a
  controller supports only a single message. I haven't seen such an adapter
  out in the wild, though, so this change likely is a NOP.
  While at it, further simplify the MSI allocation logic; there's no need
  to check the number of available messages on our own as pci_alloc_msi(9)
  will just fail if it can't provide us with the single message we want.
- Nuke the unused softc of aacch(4).
MFC r255273,255282:

Align stacks of kernel threads correctly at 16-byte boundaries rather than
making sure they are all misaligned at +8 bytes. This fixes clang builds
of powerpc64 kernels (aside from a required increase in KSTACK_PAGES which
will come later).
release/9.2.0 has been created; call stable/9 'STABLE' again.
Update for when we hope to have 9.2-RELEASE ready.
MFC 249028:

The CTL block backend is broken without this.

  Fix sending virtual scatter/gather lists from the CTL CAM frontend
  peripheral.

  Sponsored by:	Spectra Logic
Merge remote-tracking branch 'upstream-github/stable/9' into stable/9