target: Fix zero-length READ_CAPACITY_16 regression

This patch fixes a regression introduced in v3.8-rc1 code where a
zero-length READ_CAPACITY_16 was no longer returning GOOD status, but
instead returning TCM_LOGICAL_UNIT_COMMUNICATION_FAILURE to generate
a CHECK_CONDITION status.

This regression was introduced with the following commit:

  commit de103c93
  Author: Christoph Hellwig <hch@lst.de>
  Date:   Tue Nov 6 12:24:09 2012 -0800

      target: pass sense_reason as a return value

and this patch has been tested with the following zero-length CDB:

  sg_raw /dev/sdd 9e 10 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  SCSI Status: Good

  Sense Information:
  sense buffer empty

Also, convert sbc_emulate_readcapacity() to follow the same method
of handling transport_kmap_data_sg() return values, but we never
expect a zero-length request here.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Roland Dreier <roland@purestorage.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>