net/dns: Fix DNS answer RR computation

- Remove a routine that erroneously computes an answer RR value:
  dns_answer_rdlength.

- Change: dns_unpack_answer_rdlength -> dns_answer_rdlength.

- Change: htonl -> ntohl in the dns_answer_ttl routine.

Change-Id: I8b5a16f63560d3e23dd38cb7339b6e232bb706ea
Signed-off-by: Flavio Santes <flavio.santes@intel.com>