fs/resctrl: Report invalid domain ID when parsing io_alloc_cbm

fs/resctrl: Report invalid domain ID when parsing io_alloc_cbm

The last_cmd_status file is intended to report details about the most recent
resctrl filesystem operation, specifically to aid in diagnosing failures.

However, when parsing io_alloc_cbm, if a user provides a domain ID that does
not exist in the resource, the operation fails with -EINVAL without updating
last_cmd_status. This results in inconsistent behaviour where the system call
returns an error, but last_cmd_status misleadingly reports "ok", leaving the
user unaware that the failure was caused by an invalid domain ID.

Write an error message to last_cmd_status when the target domain ID cannot
be found.

Fixes: 28fa2cce7a83 ("fs/resctrl: Introduce interface to modify io_alloc capacity bitmasks")
Suggested-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Reviewed-by: Babu Moger <babu.moger@amd.com>
Tested-by: Babu Moger <babu.moger@amd.com>
Link: https://patch.msgid.link/20260325001159.447075-2-atomlin@atomlin.com