netprio_cgroup: shorten variable names in extend_netdev_table()

The function is about to go through a rewrite.  In preparation,
shorten the variable names so that we don't repeat "priomap" so often.

This patch is cosmetic.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Tested-and-Acked-by: Daniel Wagner <daniel.wagner@bmw-carit.de>
Acked-by: David S. Miller <davem@davemloft.net>