arch/arm64: Fix topology initialization for core scheduling

In the Linux kernel, the following vulnerability has been resolved:

arch/arm64: Fix topology initialization for core scheduling

Arm64 systems rely on store_cpu_topology() to call update_siblings_masks()

to transfer the toplogy to the various cpu masks. This needs to be done

before the call to notify_cpu_starting() which tells the scheduler about

each cpu found, otherwise the core scheduling data structures are setup

in a way that does not match the actual topology.

With smt_mask not setup correctly we bail on `cpumask_weight(smt_mask) == 1`

for !leaders in:

notify_cpu_starting()

cpuhp_invoke_callback_range()

sched_cpu_starting()

sched_core_cpu_starting()

which leads to rq->core not being correctly set for !leader-rq's.

Without this change stress-ng (which enables core scheduling in its prctl

tests in newer versions -- i.e. with PR_SCHED_CORE support) causes a warning

and then a crash (trimmed for legibility):

[ 1853.805168] ------------[ cut here ]------------

[ 1853.809784] task_rq(b)->core != rq->core

[ 1853.809792] WARNING: CPU: 117 PID: 0 at kernel/sched/fair.c:11102 cfs_prio_less+0x1b4/0x1c4

...

[ 1854.015210] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000010

...

[ 1854.231256] Call trace:

[ 1854.233689] pick_next_task+0x3dc/0x81c

[ 1854.237512] __schedule+0x10c/0x4cc

[ 1854.240988] schedule_idle+0x34/0x54