skbuff: move __alloc_skb() next to the other skb allocation functions

In preparation before reusing several functions in all three skb
allocation variants, move __alloc_skb() next to the
__netdev_alloc_skb() and __napi_alloc_skb().
No functional changes.

Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Signed-off-by: David S. Miller <davem@davemloft.net>