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
Reviewed by: allanjude, sef
Approved by: re (kib)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D17523