IB/core: Create get_perf_mad function in sysfs.c

Create a new function to retrieve performance management
data from the existing code in get_pma_counter().

Reviewed-by: Hal Rosenstock <hal@mellanox.com>
Signed-off-by: Christoph Lameter <cl@linux.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>