Btrfs: free space cache cleanups

This patch cleans up the free space cache code a bit.  It better documents the
idiosyncrasies of tree_search_offset and makes the code make a bit more sense.
I took out the info allocation at the start of __btrfs_add_free_space and put it
where it makes more sense.  This was left over cruft from when alloc_mutex
existed.  Also all of the re-searches we do to make sure we inserted properly.

Signed-off-by: Josef Bacik <jbacik@redhat.com>

fs/btrfs/extent-tree.c

@@ -291,8 +291,8 @@ next:
 			   block_group->key.objectid +
 			   block_group->key.offset);
 
-	remove_sb_from_cache(root, block_group);
 	block_group->cached = 1;
+	remove_sb_from_cache(root, block_group);
 	ret = 0;
 err:
 	btrfs_free_path(path);