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

This reverts commit 70afa3998c9baed4186df38988246de1abdab56d.  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>

fs/btrfs/extent-tree.c

@@ -3925,7 +3925,6 @@ static int can_overcommit(struct btrfs_root *root,
 	u64 space_size;
 	u64 avail;
 	u64 used;
-	u64 to_add;
 
 	used = space_info->bytes_used + space_info->bytes_reserved +
 		space_info->bytes_pinned + space_info->bytes_readonly;
@@ -3959,25 +3958,17 @@ static int can_overcommit(struct btrfs_root *root,
 		       BTRFS_BLOCK_GROUP_RAID10))
 		avail >>= 1;
 
-	to_add = space_info->total_bytes;
-
 	/*
 	 * If we aren't flushing all things, let us overcommit up to
 	 * 1/2th of the space. If we can flush, don't let us overcommit
 	 * too much, let it overcommit up to 1/8 of the space.
 	 */
 	if (flush == BTRFS_RESERVE_FLUSH_ALL)
-		to_add >>= 3;
+		avail >>= 3;
 	else
-		to_add >>= 1;
-
-	/*
-	 * Limit the overcommit to the amount of free space we could possibly
-	 * allocate for chunks.
-	 */
-	to_add = min(avail, to_add);
+		avail >>= 1;
 
-	if (used + bytes < space_info->total_bytes + to_add)
+	if (used + bytes < space_info->total_bytes + avail)
 		return 1;
 	return 0;
 }