arm64: clear_page() shouldn't use DC ZVA when DCZID_EL0.DZP == 1

主线补丁回合：
```diff
commit f0616abd4e67143b45b04b565839148458857347
Author: Reiji Watanabe <reijiw@google.com>
Date:   Sun Dec 5 16:47:35 2021 -0800

arm64: clear_page() shouldn't use DC ZVA when DCZID_EL0.DZP == 1
    
    Currently, clear_page() uses DC ZVA instruction unconditionally.  But it
    should make sure that DCZID_EL0.DZP, which indicates whether or not use
    of DC ZVA instruction is prohibited, is zero when using the instruction.
    Use STNP instead when DCZID_EL0.DZP == 1.
    
    Fixes: f27bb139c387 ("arm64: Miscellaneous library functions")
    Signed-off-by: Reiji Watanabe <reijiw@google.com>
    Reviewed-by: Robin Murphy <robin.murphy@arm.com>
    Link: https://lore.kernel.org/r/20211206004736.1520989-2-reijiw@google.com
    Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

diff --git a/arch/arm64/lib/clear_page.S b/arch/arm64/lib/clear_page.S
index b84b179edba3..1fd5d790ab80 100644
--- a/arch/arm64/lib/clear_page.S
+++ b/arch/arm64/lib/clear_page.S
@@ -16,6 +16,7 @@
  */
 SYM_FUNC_START_PI(clear_page)
 	mrs	x1, dczid_el0
+	tbnz	x1, #4, 2f	/* Branch if DC ZVA is prohibited */
 	and	w1, w1, #0xf
 	mov	x2, #4
 	lsl	x1, x2, x1
@@ -25,5 +26,14 @@ SYM_FUNC_START_PI(clear_page)
 	tst	x0, #(PAGE_SIZE - 1)
 	b.ne	1b
 	ret
+
+2:	stnp	xzr, xzr, [x0]
+	stnp	xzr, xzr, [x0, #16]
+	stnp	xzr, xzr, [x0, #32]
+	stnp	xzr, xzr, [x0, #48]
+	add	x0, x0, #64
+	tst	x0, #(PAGE_SIZE - 1)
+	b.ne	2b
+	ret
 SYM_FUNC_END_PI(clear_page)
 EXPORT_SYMBOL(clear_page)
```

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