xfs: Simplify xfs_ail_min() with list_first_entry_or_null()

At xfs_ail_min(), we do check if the AIL list is empty or not before
returning the first item in it with list_empty() and list_first_entry().

This can be simplified a bit with a new list operation routine that is
the list_first_entry_or_null() which has been introduced by:

commit 6d7581e6
    list: introduce list_first_entry_or_null

v2: make xfs_ail_min() as a static inline function and move it to
    xfs_trans_priv.h as per Dave Chinner's comments.
Signed-off-by: Jie Liu <jeff.liu@oracle.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

fs/xfs/xfs_trans_ail.c

@@ -61,20 +61,6 @@ xfs_ail_check(
 #endif /* DEBUG */
 
 /*
- * Return a pointer to the first item in the AIL.  If the AIL is empty, then
- * return NULL.
- */
-xfs_log_item_t *
-xfs_ail_min(
-	struct xfs_ail  *ailp)
-{
-	if (list_empty(&ailp->xa_ail))
-		return NULL;
-
-	return list_first_entry(&ailp->xa_ail, xfs_log_item_t, li_ail);
-}
-
- /*
  * Return a pointer to the last item in the AIL.  If the AIL is empty, then
  * return NULL.
  */

fs/xfs/xfs_trans_priv.h

@@ -86,6 +86,18 @@ void	xfs_trans_ail_update_bulk(struct xfs_ail *ailp,
 				struct xfs_ail_cursor *cur,
 				struct xfs_log_item **log_items, int nr_items,
 				xfs_lsn_t lsn) __releases(ailp->xa_lock);
+/*
+ * Return a pointer to the first item in the AIL.  If the AIL is empty, then
+ * return NULL.
+ */
+static inline struct xfs_log_item *
+xfs_ail_min(
+	struct xfs_ail  *ailp)
+{
+	return list_first_entry_or_null(&ailp->xa_ail, struct xfs_log_item,
+					li_ail);
+}
+
 static inline void
 xfs_trans_ail_update(
 	struct xfs_ail		*ailp,