arm64: tlb: Ensure we execute an ISB following walk cache invalidation

05f2d2f8 ("arm64: tlbflush: Introduce __flush_tlb_kernel_pgtable")
added a new TLB invalidation helper which is used when freeing
intermediate levels of page table used for kernel mappings, but is
missing the required ISB instruction after completion of the TLBI
instruction.

Add the missing barrier.

Cc: <stable@vger.kernel.org>
Fixes: 05f2d2f8

 ("arm64: tlbflush: Introduce __flush_tlb_kernel_pgtable")
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>