fs/resctrl: Introduce the interface to modify assignments in a group

fs/resctrl: Introduce the interface to modify assignments in a group

Enable the mbm_l3_assignments resctrl file to be used to modify counter
assignments of CTRL_MON and MON groups when the "mbm_event" counter
assignment mode is enabled.

Process the assignment modifications in the following format:
<Event>:<Domain id>=<Assignment state>;<Domain id>=<Assignment state>

Event: A valid MBM event in the
       /sys/fs/resctrl/info/L3_MON/event_configs directory.

Domain ID: A valid domain ID. When writing, '*' applies the changes
	   to all domains.

Assignment states:

    _ : Unassign a counter.

    e : Assign a counter exclusively.

Examples:

  $ cd /sys/fs/resctrl
  $ cat /sys/fs/resctrl/mbm_L3_assignments
    mbm_total_bytes:0=e;1=e
    mbm_local_bytes:0=e;1=e

To unassign the counter associated with the mbm_total_bytes event on
domain 0:

  $ echo "mbm_total_bytes:0=_" > mbm_L3_assignments
  $ cat /sys/fs/resctrl/mbm_L3_assignments
    mbm_total_bytes:0=_;1=e
    mbm_local_bytes:0=e;1=e

To unassign the counter associated with the mbm_total_bytes event on
all the domains:

  $ echo "mbm_total_bytes:*=_" > mbm_L3_assignments
  $ cat /sys/fs/resctrl/mbm_L3_assignments
    mbm_total_bytes:0=_;1=_
    mbm_local_bytes:0=e;1=e

Signed-off-by: Babu Moger <babu.moger@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Link: https://lore.kernel.org/cover.1757108044.git.babu.moger@amd.com