net/smc: introduce statistics for ringbufs usage of net namespace

The buffer size histograms in smc_stats, namely rx/tx_rmbsize, record
the sizes of ringbufs for all connections that have ever appeared in
the net namespace. They are incremental and we cannot know the actual
ringbufs usage from these. So here introduces statistics for current
ringbufs usage of existing smc connections in the net namespace into
smc_stats, it will be incremented when new connection uses a ringbuf
and decremented when the ringbuf is unused.

Signed-off-by: Wen Gu <guwen@linux.alibaba.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>