icmp: Fix data-races around sysctl_icmp_echo_enable_probe.

While reading sysctl_icmp_echo_enable_probe, it can be changed
concurrently.  Thus, we need to add READ_ONCE() to its readers.

Fixes: d329ea5b ("icmp: add response to RFC 8335 PROBE messages")
Fixes: 1fd07f33 ("ipv6: ICMPV6: add response to ICMPV6 RFC 8335 PROBE messages")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>