selftests/resctrl: Split fill_buf to allow tests finer-grained control

selftests/resctrl: Split fill_buf to allow tests finer-grained control

MBM, MBA and CMT test cases call run_fill_buf() that in turn calls
fill_cache() to alloc and loop indefinitely around the buffer. This
binds buffer allocation and running the benchmark into a single bundle
so that a selftest cannot allocate a buffer once and reuse it. CAT test
doesn't want to loop around the buffer continuously and after rewrite
it needs the ability to allocate the buffer separately.

Split buffer allocation out of fill_cache() into alloc_buffer(). This
change is part of preparation for the new CAT test that allocates a
buffer and does multiple passes over the same buffer (but not in an
infinite loop).

Co-developed-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>