userns: Use kgids for sysctl_ping_group_range

- Store sysctl_ping_group_range as a paire of kgid_t values
  instead of a pair of gid_t values.
- Move the kgid conversion work from ping_init_sock into ipv4_ping_group_range
- For invalid cases reset to the default disabled state.

With the kgid_t conversion made part of the original value sanitation
from userspace understand how the code will react becomes clearer
and it becomes possible to set the sysctl ping group range from
something other than the initial user namespace.

Cc: Vasiliy Kulikov <segoon@openwall.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

init/Kconfig

@@ -948,7 +948,6 @@ config UIDGID_CONVERTED
 	depends on NETFILTER_XT_MATCH_RECENT = n
 	depends on NETFILTER_XT_TARGET_LOG = n
 	depends on NETFILTER_NETLINK_LOG = n
-	depends on INET = n
 	depends on IPV6 = n
 	depends on AF_RXRPC = n
 	depends on NET_KEY = n