This function is used to obtain a pointer to and the length of a block of
data in the netbuf buf. The arguments
data and len are result
parameters that will be filled with a pointer to the data and the length of
the data pointed to. If the netbuf is fragmented, this function gives a
pointer to one of the fragments in the netbuf. The application program must
use the fragment handling functions netbuf_first()
and netbuf_next()
in order to reach all data in the netbuf. See the example under netbuf_next()
for an example of how use netbuf_data().