skbuff: allow to use NAPI cache from __napi_alloc_skb()

{,__}napi_alloc_skb() is mostly used either for optional non-linear
receive methods (usually controlled via Ethtool private flags and off
by default) and/or for Rx copybreaks.
Use __napi_build_skb() here for obtaining skbuff_heads from NAPI cache
instead of inplace allocations. This includes both kmalloc and page
frag paths.

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