arm64: Add missing ISB after invalidating TLB in enter_vhe

arm64: Add missing ISB after invalidating TLB in enter_vhe

Although there has been a bit of back and forth on the subject, it
appears that invalidating TLBs requires an ISB instruction after the
TLBI/DSB sequence when FEAT_ETS is not implemented by the CPU.

From the bible:

  | In an implementation that does not implement FEAT_ETS, a TLB
  | maintenance instruction executed by a PE, PEx, can complete at any
  | time after it is issued, but is only guaranteed to be finished for a
  | PE, PEx, after the execution of DSB by the PEx followed by a Context
  | synchronization event

Add the missing ISB in enter_vhe(), just in case.

Fixes: f359182291c7 ("arm64: Provide an 'upgrade to VHE' stub hypercall")
Suggested-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20210224093738.3629662-4-maz@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>