netinet: default mib counter probe points off

Disable the IP/IP6/ICMP/... counter probe points by default.
They are kept enabled in debug builds, and can be enabled with
'options KDTRACE_MIB_SDT'.

Requested by: glebius
Reviewed by: glebius
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D47657