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

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>
Change-Id: If49b019942043655d3ce72021e4daa66a82c60fb
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Cyber Knight <cyberknight755@gmail.com>