selftests/resctrl: Adjust effective L3 cache size when SNC enabled

Sub-NUMA Cluster divides CPUs sharing an L3 cache into separate NUMA
nodes. Systems may support splitting into either two or four nodes.

When SNC mode is enabled the effective amount of L3 cache available
for allocation is divided by the number of nodes per L3.

Detect which SNC mode is active by comparing the number of CPUs
that share a cache with CPU0, with the number of CPUs on node0.

Reported-by: "Shaopeng Tan (Fujitsu)" <tan.shaopeng@fujitsu.com>
Closes: https://lore.kernel.org/r/TYAPR01MB6330B9B17686EF426D2C3F308B25A@TYAPR01MB6330.jpnprd01.prod.outlook.com


Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: James Morse <james.morse@arm.com>