The scst_user specification copies ioctl structures and command payloads
that predate several ABI extensions. Building a handler from those
layouts can select the wrong union member, alignment or completion path.
Commit 3fc775c75a ("EXTENDED COPY support") added remapping payloads.
Commit daee4dcb7e ("WRITE SAME command improvements") added WRITE
SAME descriptors. Commit d1bd22afd9 ("scst_user: avoid global locks")
left the explicit unregister ioctl as an obsolete no-op.
Use scst/include/scst_user.h and scst_user.c as the producer-side
source of truth. Synchronize registration, command, reply and subcommand
layouts, version checks, reserved fields and ioctl rules.
Update the fileio_tgt, stpgd and event guides from current code. Make
live-storage and event-consumption risks explicit.
Gleb Chesnokov
Vladislav Bolkhovitin