btrfs: use btrfs_read_node_slot in do_relocation

We're open coding btrfs_read_node_slot in do_relocation, replace this
with the proper helper.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>