ionic: refactor skb building

The existing ionic_rx_frags() code is a bit of a mess and can
be cleaned up by unrolling the first frag/header setup from
the loop, then reworking the do-while-loop into a for-loop.  We
rename the function to a more descriptive ionic_rx_build_skb().
We also change a couple of related variable names for readability.

Reviewed-by: Brett Creeley <brett.creeley@amd.com>
Signed-off-by: Shannon Nelson <shannon.nelson@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>