- move all of the details on offsets, lengths and buffers into a
single function instead of doing these operation from multiple places

- use a bottom up approach: try to avoid details in the high level
functions, introduce them gradually as we go deeper in the function
call stack

With helpful feedback from Jarek Poplawski.
Signed-off-by: NOctavian Purdila <opurdila@ixiacom.com>
Acked-by: NJarek Poplawski <jarkao2@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>