arm64: fix infinite stacktrace

arm64: fix infinite stacktrace

I've got this infinite stacktrace when debugging another problem:
[  908.795225] INFO: rcu_preempt detected stalls on CPUs/tasks:
[  908.796176]  1-...!: (1 GPs behind) idle=952/1/4611686018427387904 softirq=1462/1462 fqs=355
[  908.797692]  2-...!: (1 GPs behind) idle=f42/1/4611686018427387904 softirq=1550/1551 fqs=355
[  908.799189]  (detected by 0, t=2109 jiffies, g=130, c=129, q=235)
[  908.800284] Task dump for CPU 1:
[  908.800871] kworker/1:1     R  running task        0    32      2 0x00000022
[  908.802127] Workqueue: writecache-writeabck writecache_writeback [dm_writecache]
[  908.820285] Call trace:
[  908.824785]  __switch_to+0x68/0x90
[  908.837661]  0xfffffe00603afd90
[  908.844119]  0xfffffe00603afd90
[  908.850091]  0xfffffe00603afd90
[  908.854285]  0xfffffe00603afd90
[  908.863538]  0xfffffe00603afd90
[  908.865523]  0xfffffe00603afd90

The machine just locked up and kept on printing the same line over and
over again. This patch fixes it.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>