net: coap: fix coap packet length calculation

This calculation reads the length portion of the COAP header to determine
the length of the coap packet.  However, when encrypted via DTLS this
value seems to be getting corrupted.  Let's change this calculation so
that it will work for when DTLS is both enabled and disabled.  Use the
total length of the fragment data and substract back out the headers
to get a correct value.

Signed-off-by: Michael Scott <michael.scott@linaro.org>