arm64: tlb: Use last-level invalidation in flush_tlb_kernel_range()

flush_tlb_kernel_range() is only ever used to invalidate last-level
entries, so we can restrict the scope of the TLB invalidation
instruction.

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>