Btrfs: don't memset new tokens

Our token logic depends on token->kaddr being set, and if it is not it sets
everything properly as needed.  So instead of memsetting just set
token->kaddr to NULL.  Thanks,

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

fs/btrfs/ctree.h

@@ -1860,7 +1860,7 @@ struct btrfs_map_token {
 
 static inline void btrfs_init_map_token (struct btrfs_map_token *token)
 {
-	memset(token, 0, sizeof(*token));
+	token->kaddr = NULL;
 }
 
 /* some macros to generate set/get funcs for the struct fields.  This