x86/resctrl: Prevent NULL pointer dereference when local MBM is disabled

Booting with kernel parameter "rdt=cmt,mbmtotal,memlocal,l3cat,mba" and
executing "mount -t resctrl resctrl -o mba_MBps /sys/fs/resctrl" results in
a NULL pointer dereference on systems which do not have local MBM support
enabled..

BUG: kernel NULL pointer dereference, address: 0000000000000020
PGD 0 P4D 0
Oops: 0000 [rib#1] SMP PTI
CPU: 0 PID: 722 Comm: kworker/0:3 Not tainted 5.2.0-0.rc3.git0.1.el7_UNSUPPORTED.x86_64 rib#2
Workqueue: events mbm_handle_overflow
RIP: 0010:mbm_handle_overflow+0x150/0x2b0

Only enter the bandwith update loop if the system has local MBM enabled.

Fixes: de73f38 ("x86/intel_rdt/mba_sc: Feedback loop to dynamically update mem bandwidth")
Signed-off-by: Prarit Bhargava <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: Fenghua Yu <[email protected]>
Cc: Reinette Chatre <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: [email protected]
Link: https://lkml.kernel.org/r/[email protected]

arch/x86/kernel/cpu/resctrl/monitor.c

@@ -368,6 +368,9 @@ static void update_mba_bw(struct rdtgroup *rgrp, struct rdt_domain *dom_mbm)
 	struct list_head *head;
 	struct rdtgroup *entry;
 
+	if (!is_mbm_local_enabled())
+		return;
+
 	r_mba = &rdt_resources_all[RDT_RESOURCE_MBA];
 	closid = rgrp->closid;
 	rmid = rgrp->mon.rmid;