target: Fix xop->dbl assignment in target_xcopy_parse_segdesc_02

This patch fixes up an incorrect assignment for xop->dbl within
target_xcopy_parse_segdesc_02() code, as reported by Coverity here:

http://marc.info/?l=linux-kernel&m=137936416618490&w=2

Reported-by: Dave Jones <davej@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>