ext4: optimize ext4_should_retry_alloc() to improve ENOSPC performance

If there are no pending blocks to be released after a commit, forcing
a journal commit has no hope of helping.  It's possible that a commit
had just completed, so if there are now free blocks available for
allocation, it's worth retrying the commit.

Reported-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
diff --git a/fs/ext4/ext4_jbd2.h b/fs/ext4/ext4_jbd2.h
index 09c1ef3..b1d52c1 100644
--- a/fs/ext4/ext4_jbd2.h
+++ b/fs/ext4/ext4_jbd2.h
@@ -175,6 +175,13 @@
  * There is no guaranteed calling order of multiple registered callbacks on
  * the same transaction.
  */
+static inline void _ext4_journal_callback_add(handle_t *handle,
+			struct ext4_journal_cb_entry *jce)
+{
+	/* Add the jce to transaction's private list */
+	list_add_tail(&jce->jce_list, &handle->h_transaction->t_private_list);
+}
+
 static inline void ext4_journal_callback_add(handle_t *handle,
 			void (*func)(struct super_block *sb,
 				     struct ext4_journal_cb_entry *jce,
@@ -187,10 +194,11 @@
 	/* Add the jce to transaction's private list */
 	jce->jce_func = func;
 	spin_lock(&sbi->s_md_lock);
-	list_add_tail(&jce->jce_list, &handle->h_transaction->t_private_list);
+	_ext4_journal_callback_add(handle, jce);
 	spin_unlock(&sbi->s_md_lock);
 }
 
+
 /**
  * ext4_journal_callback_del: delete a registered callback
  * @handle: active journal transaction handle on which callback was registered