rds: rds_rm_zerocopy_callback() use list_first_entry()

rds_rm_zerocopy_callback() uses list_entry() on the head of a list
causing a type confusion.
Use list_first_entry() to actually access the first element of the
rs_zcookie_queue list.

Fixes: 9426bbc6de99 ("rds: use list structure to track information for zerocopy completion notification")
Reviewed-by: Willem de Bruijn <[e-mail]>
Signed-off-by: Pietro Borrello <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

net/rds/message.c

···
 	spin_lock_irqsave(&q->lock, flags);
 	head = &q->zcookie_head;
 	if (!list_empty(head)) {
-		info = list_entry(head, struct rds_msg_zcopy_info,
-				  rs_zcookie_next);
-		if (info && rds_zcookie_add(info, cookie)) {
+		info = list_first_entry(head, struct rds_msg_zcopy_info,
+					rs_zcookie_next);
+		if (rds_zcookie_add(info, cookie)) {
 			spin_unlock_irqrestore(&q->lock, flags);
 			kfree(rds_info_from_znotifier(znotif));
 			/* caller invokes rds_wake_sk_sleep() */