sch_gred: prefer GFP_KERNEL allocations

In control path, its better to use GFP_KERNEL allocations where
possible.

Before taking qdisc spinlock, we preallocate memory just in case we'll
need it in gred_change_vq()

This is a followup to commit 3f1e6d3fd37b (sch_gred: should not use
GFP_KERNEL while holding a spinlock)

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>