btrfs: struct reada_control.root -> reada_control.fs_info

The root is never used.  We substitute extent_root in for the
reada_find_extent call, since it's only ever used to obtain the node
size.  This call site will be changed to use fs_info in a later patch.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>