Target/sbc: Initialize COMPARE_AND_WRITE write_sg scatterlist

When compiled with CONFIG_DEBUG_SG set, uninitialized SGL leads
to BUG() in compare_and_write_callback().

Signed-off-by: Martin Svec <martin.svec@zoner.cz>
Cc: <stable@vger.kernel.org> #3.12+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>