bhyve/virtio-scsi: Don't invoke iov_to_buf() in an assert() expression

[rosenfeld_grumpf.hope-2000.org]

Summary

If anyone would build bhyve with -DNDEBUG, any code in the expression
in assert() won't be executed. Remove the assertion and check the
returned size from iov_to_buf() explicitly, exiting if it is wrong.

[emaste]

usr.sbin/bhyve/pci_virtio_scsi.c

753

[rosenfeld_grumpf.hope-2000.org]

Review from emaste.

[des]

usr.sbin/bhyve/pci_virtio_scsi.c

751

This still fails to build with gcc.

[rosenfeld_grumpf.hope-2000.org]

usr.sbin/bhyve/pci_virtio_scsi.c

751

That's not surprising, given this is a fix for a different issue. Please see https://reviews.freebsd.org/D55800 for the proper fix of the build failure with gcc.

[rosenfeld_grumpf.hope-2000.org]

Rebase.

[emaste]

Or, something along the lines of:

size_t res __maybe_unused;
res = iov_to_buf(...);
assert (res == VTSCSI_IN_HEADER_LEN(q->vsq_sc);