target: support zero allocation length in SBC commands

READ CAPACITY must be subject to the same treatment as INQUIRY,
REQUEST SENSE, and MODE SENSE, but there are no pre-existing bugs
to fix here.  Just use an on-stack buffer, and copy to it after
checking the return value of transport_kmap_data_sg.

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