tcp_metrics: tcpm_hash_bucket is strictly local

After commit 098a697b ("tcp_metrics: Use a single hash table
for all network namespaces."), tcpm_hash_bucket is local to
net/ipv4/tcp_metrics.c

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>