ext4: Replace hackish ext4_mb_poll_new_transaction with commit callback

The multiblock allocator needs to be able to release blocks (and issue
a blkdev discard request) when the transaction which freed those
blocks is committed.  Previously this was done via a polling mechanism
when blocks are allocated or freed.  A much better way of doing things
is to create a jbd2 callback function and attaching the list of blocks
to be freed directly to the transaction structure.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

include/linux/jbd2.h

@@ -641,6 +641,11 @@ struct transaction_s
 	 */
 	int t_handle_count;
 
+	/*
+	 * For use by the filesystem to store fs-specific data
+	 * structures associated with the transaction
+	 */
+	struct list_head	t_private_list;
 };
 
 struct transaction_run_stats_s {
@@ -935,6 +940,10 @@ struct journal_s
 
 	pid_t			j_last_sync_writer;
 
+	/* This function is called when a transaction is closed */
+	void			(*j_commit_callback)(journal_t *,
+						     transaction_t *);
+
 	/*
 	 * Journal statistics
 	 */