UBIFS: remove invalid reference to list iterator variable

If list_for_each_entry, etc complete a traversal of the list, the iterator
variable ends up pointing to an address at an offset from the list head,
and not a meaningful structure.  Thus this value should not be used after
the end of the iterator.  Replace a field access from orphan by NULL in two
places.

A simplified version of the semantic match that finds this problem is as
follows: (http://coccinelle.lip6.fr/)

// <smpl>
@@
identifier c;
expression E;
iterator name list_for_each_entry;
statement S;
@@

list_for_each_entry(c,...) { ... when != break;
                                 when forall
                                 when strict
}
...
(
c = E
|
*c
)
// </smpl>

Artem: fortunately, this did not cause any issues because we iterate the orphan
list using the elements count, so we never dereferenced the corrupted pointer.
This is why I do not send this patch to -stable. But otherwise - well spotted!
Signed-off-by: NJulia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: NArtem Bityutskiy <Artem.Bityutskiy@linux.intel.com>

fs/ubifs/orphan.c

@@ -176,7 +176,7 @@ int ubifs_orphan_start_commit(struct ubifs_info *c)
 		*last = orphan;
 		last = &orphan->cnext;
 	}
-	*last = orphan->cnext;
+	*last = NULL;
 	c->cmt_orphans = c->new_orphans;
 	c->new_orphans = 0;
 	dbg_cmt("%d orphans to commit", c->cmt_orphans);
@@ -382,7 +382,7 @@ static int consolidate(struct ubifs_info *c)
 			last = &orphan->cnext;
 			cnt += 1;
 		}
-		*last = orphan->cnext;
+		*last = NULL;
 		ubifs_assert(cnt == c->tot_orphans - c->new_orphans);
 		c->cmt_orphans = cnt;
 		c->ohead_lnum = c->orph_first;