Clearing the way for ext4 snapshots

Hi Ted,

I pushed a few small patches today, all are pre-snapshot patches,
which I hope you can merge in coming merge window.

These 2, are the only snapshot patches outside fs/ext4:

[PATCH 1/2] jbd2: add the b_cow_tid field to journal_head struct 						
[PATCH 2/2] jbd2: add the h_cowing flag and debug counters to
jbd2_journal_handle struct

Merging them soon will allow early ext4 snapshots testers to build a standalone
module, without having to patch and build the kernel.

These 2, are no-brainer error handling.
Since they are not an integral part of the snapshot patches,
I rather merge them sooner and not clutter the snapshot patches
review with unrelated stuff.

[PATCH 1/2] ext4: handle errors in ext4_rename 						
[PATCH 2/2] ext4: handle errors in ext4_clear_blocks() 						

This one is also a no-brainer cleanup, which will be required by snapshots.

[PATCH] ext4: fix callers of ext4_handle_release_buffer() API 						

Thanks,
Amir.