arm_mpam: Fix null pointer dereference when restoring bandwidth counters

When an MSC supporting memory bandwidth monitoring is brought offline and
then online, mpam_restore_mbwu_state() calls __ris_msmon_read() via ipi to
restore the configuration of the bandwidth counters. It doesn't care about
the value read, mbwu_arg.val, and doesn't set it leading to a null pointer
dereference when __ris_msmon_read() adds to it. This results in a kernel
oops with a call trace such as:

Call trace:
__ris_msmon_read+0x19c/0x64c (P)
mpam_restore_mbwu_state+0xa0/0xe8
smp_call_on_cpu_callback+0x1c/0x38
process_one_work+0x154/0x4b4
worker_thread+0x188/0x310
kthread+0x11c/0x130
ret_from_fork+0x10/0x20

Provide a local variable for val to avoid __ris_msmon_read() dereferencing
a null pointer when adding to val.

Fixes: 41e8a14 ("arm_mpam: Track bandwidth counter state for power management")
Signed-off-by: Ben Horgan <ben.horgan@arm.com>
Reviewed-by: James Morse <james.morse@arm.com>
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>

Author: benhor01

drivers/resctrl/mpam_devices.c

@@ -1428,6 +1428,7 @@ static void mpam_reprogram_ris_partid(struct mpam_msc_ris *ris, u16 partid,
 static int mpam_restore_mbwu_state(void *_ris)
 {
 	int i;
+	u64 val;
 	struct mon_read mwbu_arg;
 	struct mpam_msc_ris *ris = _ris;
 	struct mpam_class *class = ris->vmsc->comp->class;
@@ -1437,6 +1438,7 @@ static int mpam_restore_mbwu_state(void *_ris)
 			mwbu_arg.ris = ris;
 			mwbu_arg.ctx = &ris->mbwu_state[i].cfg;
 			mwbu_arg.type = mpam_msmon_choose_counter(class);
+			mwbu_arg.val = &val;
 
 			__ris_msmon_read(&mwbu_arg);
 		}