btrfs: use list_last_entry in add_falloc_range

Instead of calling list_entry with head->prev simply call
list_last_entry which makes it obvious which member of the list is
being referred. This allows to remove the extra 'prev' pointer.
Signed-off-by: NNikolay Borisov <nborisov@suse.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/file.c

@@ -3034,7 +3034,6 @@ struct falloc_range {
  */
 static int add_falloc_range(struct list_head *head, u64 start, u64 len)
 {
-	struct falloc_range *prev = NULL;
 	struct falloc_range *range = NULL;
 
 	if (list_empty(head))
@@ -3044,9 +3043,9 @@ static int add_falloc_range(struct list_head *head, u64 start, u64 len)
 	 * As fallocate iterate by bytenr order, we only need to check
 	 * the last range.
 	 */
-	prev = list_entry(head->prev, struct falloc_range, list);
-	if (prev->start + prev->len == start) {
-		prev->len += len;
+	range = list_last_entry(head, struct falloc_range, list);
+	if (range->start + range->len == start) {
+		range->len += len;
 		return 0;
 	}
 insert: