netprio_cgroup: Fix obo in get_prioidx

It was recently pointed out to me that the get_prioidx function sets a bit in
the prioidx map prior to checking to see if the index being set is out of
bounds.  This patch corrects that, avoiding the possiblity of us writing beyond
the end of the array

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Reported-by: Stanislaw Gruszka <sgruszka@redhat.com>
CC: Stanislaw Gruszka <sgruszka@redhat.com>
CC: "David S. Miller" <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>