KVM: arm64: Return early from trace helpers when KVM isn't available

KVM: arm64: Return early from trace helpers when KVM isn't available

When Linux is booted at EL1, host_data_ptr() resolves to the nVHE
hypervisor's copy of host data. When hyp mode isn't available for
KVM the nVHE percpu bases remain uninitialized. Consequently, any usage
of host_data_ptr() will result in a NULL dereference which has been
observed in KVM's trace filtering helpers.

Add an early return to the trace filtering helpers if KVM isn't
initialized, avoiding the NULL dereference. Take this opportunity
to move the TRBE-skipping checks to a common helper.

Fixes: 054b88391bbe2 ("KVM: arm64: Support trace filtering for guests")
Signed-off-by: Yingchao Deng <yingchao.deng@oss.qualcomm.com>
Reviewed-by: James Clark <james.clark@linaro.org>
[maz: repainted the helpers to be readable, and the commit message
 with Oliver's suggestion]
Signed-off-by: Marc Zyngier <maz@kernel.org>