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>

drivers/target/target_core_sbc.c

@@ -425,13 +425,14 @@ static sense_reason_t compare_and_write_callback(struct se_cmd *cmd)
 		goto out;
 	}
 
-	write_sg = kzalloc(sizeof(struct scatterlist) * cmd->t_data_nents,
+	write_sg = kmalloc(sizeof(struct scatterlist) * cmd->t_data_nents,
 			   GFP_KERNEL);
 	if (!write_sg) {
 		pr_err("Unable to allocate compare_and_write sg\n");
 		ret = TCM_OUT_OF_RESOURCES;
 		goto out;
 	}
+	sg_init_table(write_sg, cmd->t_data_nents);
 	/*
 	 * Setup verify and write data payloads from total NumberLBAs.
 	 */