Btrfs: Use ERR_CAST helpers

Use ERR_CAST(x) rather than ERR_PTR(PTR_ERR(x)).  The former makes more
clear what is the purpose of the operation, which otherwise looks like a
no-op.

The semantic patch that makes this change is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@@
type T;
T x;
identifier f;
@@

T f (...) { <+...
- ERR_PTR(PTR_ERR(x))
+ x
 ...+> }

@@
expression x;
@@

- ERR_PTR(PTR_ERR(x))
+ ERR_CAST(x)
// </smpl>
Signed-off-by: Julia Lawall <julia@diku.dk>
Cc: Chris Mason <chris.mason@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/extent_map.c

@@ -335,7 +335,7 @@ struct extent_map *lookup_extent_mapping(struct extent_map_tree *tree,
 		goto out;
 	}
 	if (IS_ERR(rb_node)) {
-		em = ERR_PTR(PTR_ERR(rb_node));
+		em = ERR_CAST(rb_node);
 		goto out;
 	}
 	em = rb_entry(rb_node, struct extent_map, rb_node);
@@ -384,7 +384,7 @@ struct extent_map *search_extent_mapping(struct extent_map_tree *tree,
 		goto out;
 	}
 	if (IS_ERR(rb_node)) {
-		em = ERR_PTR(PTR_ERR(rb_node));
+		em = ERR_CAST(rb_node);
 		goto out;
 	}
 	em = rb_entry(rb_node, struct extent_map, rb_node);

fs/btrfs/super.c

@@ -389,7 +389,7 @@ static struct dentry *get_default_root(struct super_block *sb,
 find_root:
 	new_root = btrfs_read_fs_root_no_name(root->fs_info, &location);
 	if (IS_ERR(new_root))
-		return ERR_PTR(PTR_ERR(new_root));
+		return ERR_CAST(new_root);
 
 	if (btrfs_root_refs(&new_root->root_item) == 0)
 		return ERR_PTR(-ENOENT);