target: support zero-size allocation lengths in transport_kmap_data_sg

In order to support zero-size allocation lengths, do not assert
that we have a scatterlist until after checking cmd->data_length.

But once we do this, we can have two cases of transport_kmap_data_sg
returning NULL: a zero-size allocation length, or an out-of-memory
condition.  Report the latter using sense codes, so that the SCSI
command that triggered it will fail.

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