arm64: fix KASLR boot-time I-cache maintenance

Commit f80fb3a3 ("arm64: add support for kernel ASLR") missed a
DSB necessary to complete I-cache maintenance in the primary boot path,
and hence stale instructions may still be present in the I-cache and may
be executed until the I-cache maintenance naturally completes.

Since commit 8ec41987 ("arm64: mm: ensure patched kernel text is
fetched from PoU"), all CPUs invalidate their I-caches after their MMU
is enabled. Prior a CPU's MMU having been enabled, arbitrary lines may
have been fetched from the PoC into I-caches. We never patch text
expected to be executed with the MMU off. Thus, it is unnecessary to
perform broadcast I-cache maintenance in the primary boot path.

This patch reduces the scope of the I-cache maintenance to the local
CPU, and adds the missing DSB with similar scope, matching prior
maintenance in the primary boot path.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Ard Biesehvuel <ard.biesheuvel@linaro.org>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/head.S

@@ -766,8 +766,9 @@ __enable_mmu:
 
 	msr	sctlr_el1, x19			// re-enable the MMU
 	isb
-	ic	ialluis				// flush instructions fetched
-	isb					// via old mapping
+	ic	iallu				// flush instructions fetched
+	dsb	nsh				// via old mapping
+	isb
 	add	x27, x27, x23			// relocated __mmap_switched
 #endif
 	br	x27