arm64: Update comment for ASID() macro

Commit 25b92693 ("arm64: mm: convert cpu_do_switch_mm() to C") added
a new use of the ASID() macro, so update the comment in asm/mmu.h which
reasons about why an atomic reload of 'mm->context.id.counter' is not
required.

Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/mmu.h

@@ -23,9 +23,9 @@ typedef struct {
 } mm_context_t;
 
 /*
- * This macro is only used by the TLBI code, which cannot race with an
- * ASID change and therefore doesn't need to reload the counter using
- * atomic64_read.
+ * This macro is only used by the TLBI and low-level switch_mm() code,
+ * neither of which can race with an ASID change. We therefore don't
+ * need to reload the counter using atomic64_read().
  */
 #define ASID(mm)	((mm)->context.id.counter & 0xffff)