net: add napi_get_frags_check() helper

This is a follow up of commit 3226b158


("net: avoid 32 x truesize under-estimation for tiny skbs")

When/if we increase MAX_SKB_FRAGS, we better make sure
the old bug will not come back.

Adding a check in napi_get_frags() would be costly,
even if using DEBUG_NET_WARN_ON_ONCE().

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>