net/smc: introduce statistics for allocated ringbufs of link group

net/smc: introduce statistics for allocated ringbufs of link group

Currently we have the statistics on sndbuf/RMB sizes of all connections
that have ever been on the link group, namely smc_stats_memsize. However
these statistics are incremental and since the ringbufs of link group
are allowed to be reused, we cannot know the actual allocated buffers
through these. So here introduces the statistic on actual allocated
ringbufs of the link group, it will be incremented when a new ringbuf is
added into buf_list and decremented when it is deleted from buf_list.

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