perf/mem: Rename PERF_MEM_LVLNUM_EXTN_MEM to PERF_MEM_LVLNUM_CXL

PERF_MEM_LVLNUM_EXTN_MEM was introduced to cover CXL devices but it's
bit ambiguous name and also not generic enough to cover cxl.cache and
cxl.io devices. Rename it to PERF_MEM_LVLNUM_CXL to be more specific.

Signed-off-by: Ravi Bangoria <ravi.bangoria@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/f6268268-b4e9-9ed6-0453-65792644d953@amd.com