fs/resctrl: Communicate resource group deleted error via last_cmd_status

User space expects last_cmd_status to contain additional information if any
resctrl command fails.

A resctrl command may be blocked on the rdtgroup_mutex waiting for another
command to finish and find that once the mutex is available that the resource
group has since been deleted.

In this scenario the command will fail while last_cmd_status contains either
"ok" if the last_cmd_status buffer is empty or an outdated error from
a previous command failure if last_cmd_status buffer has content.

Include clearing of last_cmd_status buffer as part of rdtgroup_kn_lock_live()
that is used to obtain access and needed locking to a resource group before
attempting a command on the group.

With the last_cmd_status buffer ready, provide an appropriate message to user
space if the resource group has been deleted.

No last_cmd_status treatment is needed for the remaining failure of
rdtgroup_kn_lock_live() encountering a non-existent resource group since that
could only occur during an attempt to obtain a resource group lock on a file
in info/ which is an invalid usage.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Tested-by: Babu Moger <babu.moger@amd.com>
Link: https://patch.msgid.link/9982141ec6f3ec18e0c53d7feabb19651583cf0e.1782857711.git.reinette.chatre@intel.com

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