sctp: count singleton chunks in assoc user stats

Singleton chunks (INIT, HEARTBEAT PMTU probes, and SHUTDOWN-
COMPLETE) are not counted in SCTP_GET_ASOC_STATS "sas_octrlchunks"
counter available to the assoc owner.

These are all control chunks so they should be counted as such.

Add counting of singleton chunks so they are properly accounted for.

Fixes: 196d6759

 ("sctp: Add support to per-association statistics via a new SCTP_GET_ASSOC_STATS call")
Signed-off-by: Jamie Bainbridge <jamie.bainbridge@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Link: https://lore.kernel.org/r/c9ba8785789880cf07923b8a5051e174442ea9ee.1649029663.git.jamie.bainbridge@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>