rpl: wrong length for DAO options

similar to #2216
According to RFC 6550, option `length` fields should not contain the two
bytes derived from the `length` field itself and the option `type` field.

Furthermore, the current implementation adds the parent's address (16
bytes) to transit options `length`, although this field is not used in
storing mode.
I subtracted the parennt's address length from the transit options
length for the storing mode.

Here again, can be reproduced by running the rpl_udp example and logging
DAOs via wireshark.