The offset was not bumped back to the full size after writing the
header of the MODE SENSE response, so the last 1 or 2 bytes were
not copied.

On top of this, support zero-length requests by checking for the
return value of transport_kmap_data_sg.

Testcase: sg_raw -r20 /dev/sdb 5a 00 0a 00 00 00 00 00 14 00
    last byte should be 0x1e
    it is 0x00 without the patch
    it is correct with the patch
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>