ext3: Avoid filesystem corruption after a crash under heavy delete load

It can happen that ext3_free_branches calls ext3_forget() for an indirect block
in an earlier transaction than a transaction in which we clear pointer to this
indirect block. Thus if we crash before a transaction clearing the block
pointer is committed, we will see indirect block pointing to already freed
blocks and complain during orphan list cleanup.

The fix is simple: Make sure ext3_forget() is called in the transaction
doing block pointer clearing.

This is a backport of an ext4 fix by Amir G. <amir73il@users.sourceforge.net>

Signed-off-by: Jan Kara <jack@suse.cz>