MFC r339329: Add ZIO_TYPE_FREE support for indirect vdevs.
Upstream code expects only ZIO_TYPE_READ and some ZIO_TYPE_WRITE
requests to removed (indirect) vdevs, while on FreeBSD there is also
ZIO_TYPE_FREE (TRIM). ZIO_TYPE_FREE requests do not have the data
buffers, so don't need the pointer adjustment.
PR: 228750, 229007