net: ping: small changes

ping_table is not __read_mostly, since it contains one rwlock,
and is static to ping.c

ping_port_rover & ping_v4_lookup are static

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Vasiliy Kulikov <segoon@openwall.com>
Signed-off-by: David S. Miller <davem@davemloft.net>