Revert "Btrfs: rework the overcommit logic to be based on the total size"

This reverts commit 70afa399

.  It is causing
performance issues and wasn't actually correct.  There were problems with the
way we flushed delalloc and that was the real cause of the early enospc.
Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>