btrfs: constify ulist parameter of ulist_next()

The ulist_next() iterator function does not need to change the given ulist
so make it const. This will allow the next patch in the series to pass a
ulist to a function that does not need, and should not, modify the ulist.
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ulist.c

@@ -266,7 +266,7 @@ int ulist_del(struct ulist *ulist, u64 val, u64 aux)
  * It is allowed to call ulist_add during an enumeration. Newly added items
  * are guaranteed to show up in the running enumeration.
  */
-struct ulist_node *ulist_next(struct ulist *ulist, struct ulist_iterator *uiter)
+struct ulist_node *ulist_next(const struct ulist *ulist, struct ulist_iterator *uiter)
 {
 	struct ulist_node *node;
 

fs/btrfs/ulist.h

@@ -66,7 +66,7 @@ static inline int ulist_add_merge_ptr(struct ulist *ulist, u64 val, void *aux,
 #endif
 }
 
-struct ulist_node *ulist_next(struct ulist *ulist,
+struct ulist_node *ulist_next(const struct ulist *ulist,
 			      struct ulist_iterator *uiter);
 
 #define ULIST_ITER_INIT(uiter) ((uiter)->cur_list = NULL)