selftests/resctrl: Fix closing IMC fds on error and open-code R+W instead of loops

The imc perf fd close() calls are missing from all error paths. In
addition, get_mem_bw_imc() handles fds in a for loop but close() is
based on two fixed indexes READ and WRITE.

Open code inner for loops to READ+WRITE entries for clarity and add a
function to close() IMC fds properly in all cases.

Fixes: 7f4d257e ("selftests/resctrl: Add callback to start a benchmark")
Suggested-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Tested-by: Babu Moger <babu.moger@amd.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>