scsi: scsi_debug: Parser tables and code interaction

This patch is in response to a static analyser report from Dan Carpenter
titled: "[bug report] scsi: scsi_debug: Add per_host_store option".  This
code may not clear the static analyzer reports, but may shed light on why
they occur. Amongst other things this driver has a table driven SCSI
command parser which also involves some C code. There are some invariants
between the table entries and the corresponding C code (i.e. the resp_*()
functions) that, if broken, may lead to a NULL dereference.  And the report
is valid, at least in the case of the PRE-FETCH command.  Alas, that is not
one of the cases that the static analyzer reported.

In this particular corner case: when the fake_rw flag is set and the table
entry for a "store"-accessing command does not have the required F_FAKE_RW
flag set, do the following. Call BUG_ON() in the devip2sip() very close to
a comment block explaining why it was called and how to fix it.
checkpatch.pl complains about the BUG_ON() but there is no reasonable
remedial action that can be taken at run time.

This change allows the code reported by the static analyzer to be
simplified. Comments were also added to the table flags (e.g.  F_FAKE_RW)
so developers who add commands might be more inclined to use them
(properly).

Link: https://lore.kernel.org/r/20200513013943.25285-1-dgilbert@interlog.comReported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>