pktgen: clean up handling of local/transient counter vars

The temporary variable "i" is needlessly initialized to zero
in two distinct cases in this file:

1) where it is set to zero and then used as an argument in an addition
before being assigned a non-zero value.

2) where it is only used in a standard/typical loop counter

For (1), simply delete assignment to zero and usages while still
zero; for (2) simply make the loop start at zero as per standard
practice as seen everywhere else in the same file.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>