Add CTLFLAG_STATS to all COUNTER_U64* sysctl OIDs

CTLFLAG_STATS identifies a sysctl OID as statistical or informational,
as opposed to a configurable/tunable OID that changes behavior.
This can be used, for example, to verfiy that the kyua tests do not
modify configurable OIDs when allow_sysctl_side_effects is true.

Add CTLFLAG_STATS to all COUNTER_U64* OIDs.

I will add the flag to more OIDs in a few subsequent commits, to
facilitate MFC. The flag should be added to many more OIDs. I plan to
add it those that my test found and some nearby that looked obvious.

MFC after: 2 weeks
Sponsored by: Dell EMC Isilon