arm64: Add missing ISB after invalidating TLB in __primary_switch

Although there has been a bit of back and forth on the subject, it
appears that invalidating TLBs requires an ISB instruction 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 __primary_switch, just in case.

Fixes: 3c5e9f23 ("arm64: head.S: move KASLR processing out of __enable_mmu()")
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/20210224093738.3629662-3-maz@kernel.orgSigned-off-by: Will Deacon <will@kernel.org>

arch/arm64/kernel/head.S

@@ -837,6 +837,7 @@ SYM_FUNC_START_LOCAL(__primary_switch)
 
 	tlbi	vmalle1				// Remove any stale TLB entries
 	dsb	nsh
+	isb
 
 	set_sctlr_el1	x19			// re-enable the MMU