arm64: tlb: Rewrite stale comment in asm/tlbflush.h

mainline inclusion
from mainline-4.20-rc1
commit: 7f088727
category: feature
feature: Reduce synchronous TLB invalidation on ARM64
bugzilla: NA
CVE: NA

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Peter Z asked me to justify the barrier usage in asm/tlbflush.h, but
actually that whole block comment needs to be rewritten.

Reported-by: Peter Zijlstra <peterz@infradead.org>
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>
Signed-off-by: Hanjun Guo <guohanjun@huawei.com>
Reviewed-by: Xuefeng Wang <wxf.wang@hisilicon.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>