net: napi_hash_add() is no longer exported

There are no more users except from net/core/dev.c
napi_hash_add() can now be static.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>