iSCSI: do not use LUN id allocated on the stack

The previous patch did not box the value passed over the
FFI boundary. With optimized builds the value might get
dropped before all the callbacks where executed. In effect
a use after free.

mayastor/src/target/iscsi.rs

@@ -190,25 +190,21 @@ pub fn fini() {
 fn share_as_iscsi_target(
     bdev_name: &str,
     bdev: &Bdev,
-    pg_idx: c_int,
-    ig_idx: c_int,
+    mut pg_idx: c_int,
+    mut ig_idx: c_int,
 ) -> Result<String, Error> {
     let iqn = target_name(bdev_name).into_cstring();
 
-    let pg_tags = [pg_idx].as_mut_ptr();
-    let ig_tags = [ig_idx].as_mut_ptr();
-    let lun_ids = [0].as_mut_ptr();
-
     let tgt = unsafe {
         iscsi_tgt_node_construct(
             -1,
             iqn.as_ptr(),
             ptr::null(),
-            pg_tags,
-            ig_tags,
+            &mut pg_idx as *mut _,
+            &mut ig_idx as *mut _,
             1,
             &mut bdev.name().into_cstring().as_ptr(),
-            lun_ids,
+            &mut LUN as *mut _,
             1,
             128,
             true,