target: do not submit a zero-bio I/O request

scsi_setup_fs_cmnd does not like to receive requests with no
bios attached to it.  Special-case zero-length reads and writes,
by not submitting any bio.

Testcase: sg_raw /dev/sdb 28 00 00 00 00 00 00 00 00 00
    should not fail
    panics with the rest of the series but not this patch
    behaves correctly without or with this series

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>