selftests/resctrl: Use resctrl/info for feature detection

Resctrl test suite before running any unit test (like cmt, cat, mbm and
mba) should first check if the feature is enabled (by kernel and not just
supported by H/W) on the platform or not.
validate_resctrl_feature_request() is supposed to do that. This function
intends to grep for relevant flags in /proc/cpuinfo but there are several
issues here

1. validate_resctrl_feature_request() calls fgrep() to get flags from
   /proc/cpuinfo. But, fgrep() can only return a string with maximum of 255
   characters and hence the complete cpu flags are never returned.
2. The substring search logic is also busted. If strstr() finds requested
   resctrl feature in the cpu flags, it returns pointer to the first
   occurrence. But, the logic negates the return value of strstr() and
   hence validate_resctrl_feature_request() returns false if the feature is
   present in the cpu flags and returns true if the feature is not present.
3. validate_resctrl_feature_request() checks if a resctrl feature is
   reported in /proc/cpuinfo flags or not. Having a cpu flag means that the
   H/W supports the feature, but it doesn't mean that the kernel enabled
   it. A user could selectively enable only a subset of resctrl features
   using kernel command line arguments. Hence, /proc/cpuinfo isn't a
   reliable source to check if a feature is enabled or not.

The 3rd issue being the major one and fixing it requires changing the way
validate_resctrl_feature_request() works. Since, /proc/cpuinfo isn't the
right place to check if a resctrl feature is enabled or not, a more
appropriate place is /sys/fs/resctrl/info directory. Change
validate_resctrl_feature_request() such that,

1. For cat, check if /sys/fs/resctrl/info/L3 directory is present or not
2. For mba, check if /sys/fs/resctrl/info/MB directory is present or not
3. For cmt, check if /sys/fs/resctrl/info/L3_MON directory is present and
   check if /sys/fs/resctrl/info/L3_MON/mon_features has llc_occupancy
4. For mbm, check if /sys/fs/resctrl/info/L3_MON directory is present and
   check if /sys/fs/resctrl/info/L3_MON/mon_features has
   mbm_<total/local>_bytes

Please note that only L3_CAT, L3_CMT, MBA and MBM are supported. CDP and L2
variants can be added later.

Reported-by: Reinette Chatre <reinette.chatre@intel.com>
Tested-by: Babu Moger <babu.moger@amd.com>
Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>