ravb: Use GFP_KERNEL instead of GFP_ATOMIC when possible

'max_rx_len' can be up to GBETH_RX_BUFF_MAX (i.e. 8192) (see
'gbeth_hw_info').
The default value of 'num_rx_ring' can be BE_RX_RING_SIZE (i.e. 1024).

So this loop can allocate 8 Mo of memory.

Previous memory allocations in this function already use GFP_KERNEL, so
use __netdev_alloc_skb() and an explicit GFP_KERNEL instead of a
implicit GFP_ATOMIC.

This gives more opportunities of successful allocation.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>