arm64: Fix sampling the "stable" virtual counter in preemptible section

arm64: Fix sampling the "stable" virtual counter in preemptible section

Ben reports that when running with CONFIG_DEBUG_PREEMPT, using
__arch_counter_get_cntvct_stable() results in well deserves warnings,
as we access a per-CPU variable without preemption disabled.

Fix the issue by disabling preemption on reading the counter. We can
probably do a lot better by not disabling preemption on systems that
do not require horrible workarounds to return a valid counter value,
but this plugs the issue for the time being.

Fixes: 29cc0f3aa7c6 ("arm64: Force the use of CNTVCT_EL0 in __delay()")
Reported-by: Ben Horgan <ben.horgan@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/aZw3EGs4rbQvbAzV@e134344.arm.com
Tested-by: Ben Horgan <ben.horgan@arm.com>
Tested-by: André Draszik <andre.draszik@linaro.org>
Signed-off-by: Will Deacon <will@kernel.org>