bpf: cpumap: switch to napi_skb_cache_get_bulk()

Now that cpumap uses GRO, which drops unused skb heads to the NAPI
cache, use napi_skb_cache_get_bulk() to try to reuse cached entries
and lower MM layer pressure. Always disable the BH before checking and
running the cpumap-pinned XDP prog and don't re-enable it in between
that and allocating an skb bulk, as we can access the NAPI caches only
from the BH context.
The better GRO aggregates packets, the less new skbs will be allocated.
If an aggregated skb contains 16 frags, this means 15 skbs were returned
to the cache, so next 15 skbs will be built without allocating anything.

The same trafficgen UDP GRO test now shows:

                GRO off   GRO on
threaded GRO    2.3       4         Mpps
thr bulk GRO    2.4       4.7       Mpps

diff            +4        +17       %

Comparing to the baseline cpumap:

baseline        2.7       N/A       Mpps
thr bulk GRO    2.4       4.7       Mpps
diff            -11       +74       %

Tested-by: Daniel Xu <dxu@dxuuu.xyz>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>