net: ipv4: make the ping /proc code AF-independent

Introduce a ping_seq_afinfo structure (similar to its UDP
equivalent) and use it to make some of the ping /proc functions
address-family independent. Rename the remaining ping /proc
functions from ping_* to ping_v4_*.

Compiles and displays reasonable results with CONFIG_IPV6={n,m,y}

Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>