staging/lustre/osc: Revert erroneous list_for_each_entry_safe use

I have been having a lot of unexplainable crashes in osc_lru_shrink
lately that I could not see a good explanation for and then I found
this patch that slip under the radar somehow that incorrectly
converted while loop for lru list iteration into
list_for_each_entry_safe totally ignoring that in the body of
the loop we drop spinlocks guarding this list and move list entries
around.
Not sure why it was not showing up right away, perhaps some of the
more recent LRU changes committed caused some extra pressure on this
code that finally highlighted the breakage.

Reverts: 8adddc36b1fc ("staging: lustre: osc: Use list_for_each_entry_safe")
CC: Bhaktipriya Shridhar <[e-mail]>
Signed-off-by: Oleg Drokin <[e-mail]>
Cc: stable <[e-mail]> # 4.6+
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/lustre/lustre/osc/osc_page.c

···
 	struct cl_object *clobj = NULL;
 	struct cl_page **pvec;
 	struct osc_page *opg;
-	struct osc_page *temp;
 	int maxscan = 0;
 	long count = 0;
 	int index = 0;
···
 	if (force)
 		cli->cl_lru_reclaim++;
 	maxscan = min(target << 1, atomic_long_read(&cli->cl_lru_in_list));
-	list_for_each_entry_safe(opg, temp, &cli->cl_lru_list, ops_lru) {
+	while (!list_empty(&cli->cl_lru_list)) {
 		struct cl_page *page;
 		bool will_free = false;
 
···
 		if (--maxscan < 0)
 			break;
 
+		opg = list_entry(cli->cl_lru_list.next, struct osc_page,
+				 ops_lru);
 		page = opg->ops_cl.cpl_page;
 		if (lru_page_busy(cli, page)) {
 			list_move_tail(&opg->ops_lru, &cli->cl_lru_list);