RDMA/core: Get sum value of all counters when perform a sysfs stat read

Since a QP can only be bound to one counter, then if it is bound to a
separate counter, for backward compatibility purpose, the statistic value
must be:
* stat of default counter
+ stat of all running allocated counters
+ stat of all deallocated counters (history stats)

Signed-off-by: Mark Zhang <markz@mellanox.com>
Reviewed-by: Majd Dibbiny <majd@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>