xen-netback: Use GFP_ATOMIC to allocate hash

Allocation of new_hash, inside xenvif_new_hash(), always happen
in softirq context, so use GFP_ATOMIC instead of GFP_KERNEL for new
hash allocation.

Signed-off-by: Anoob Soman <anoob.soman@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>