target: fix NULL pointer dereference bug alloc_page() fails to get memory

I am hitting this bug when the target is low in memory that fails the
alloc_page() for the newly submitted command. This is a sort of off-by-one
bug causing NULL pointer dereference in __free_page() since 'i' here is
really the counter of total pages that have been successfully allocated here.

Signed-off-by: Yi Zou <yi.zou@intel.com>
Cc: Andy Grover <agrover@redhat.com>
Cc: Nicholas Bellinger <nab@linux-iscsi.org>
Cc: Open-FCoE.org <devel@open-fcoe.org>
Cc: stable@vger.kernel.org
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_transport.c

@@ -2294,9 +2294,9 @@ transport_generic_get_mem(struct se_cmd *cmd)
 	return 0;
 
 out:
-	while (i >= 0) {
+	while (i > 0) {
-		__free_page(sg_page(&cmd->t_data_sg[i]));
 		i--;
+		__free_page(sg_page(&cmd->t_data_sg[i]));
 	}
 	kfree(cmd->t_data_sg);
 	cmd->t_data_sg = NULL;