LU-15137 socklnd: decrement connection counters on close

LU-15137 socklnd: decrement connection counters on close

To gracefully handle potential race with delayed connection create,
decrement connection counters per type as connections are being
closed.

Test-Parameters: trivial testlist=sanity-lnet
Fixes: 71b2476e ("LU-12815 socklnd: add conns_per_peer parameter")
Signed-off-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Change-Id: Ieb3b44701e4999ea1fe63234162dd5878d65958a
Reviewed-on: https://review.whamcloud.com/45422
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Amir Shehata <ashehata@whamcloud.com>
Reviewed-by: Cyril Bordage <cbordage@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>