jffs2: fix use after free in jffs2_sum_write_data()

clang static analysis reports this problem

fs/jffs2/summary.c:794:31: warning: Use of memory after it is freed
                c->summary->sum_list_head = temp->u.next;
                                            ^~~~~~~~~~~~

In jffs2_sum_write_data(), in a loop summary data is handles a node at
a time.  When it has written out the node it is removed the summary list,
and the node is deleted.  In the corner case when a
JFFS2_FEATURE_RWCOMPAT_COPY is seen, a call is made to
jffs2_sum_disable_collecting().  jffs2_sum_disable_collecting() deletes
the whole list which conflicts with the loop's deleting the list by parts.

To preserve the old behavior of stopping the write midway, bail out of
the loop after disabling summary collection.

Fixes: 6171586a ("[JFFS2] Correct handling of JFFS2_FEATURE_RWCOMPAT_COPY nodes.")
Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>