net/ipv4: Use non-atomic allocation of udp offloads structure instance

Since udp_add_offload() can be called from non-sleepable context e.g
under this call tree from the vxlan driver use case:

  vxlan_socket_create() <-- holds the spinlock
  -> vxlan_notify_add_rx_port()
     -> udp_add_offload()  <-- schedules

we should allocate the udp_offloads structure in atomic manner.

Fixes: b582ef09 ('net: Add GRO support for UDP encapsulating protocols')
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>