arm64: add DSB after icache flush in __flush_icache_all()

Add DSB after icache flush to complete the cache maintenance operation.
The function __flush_icache_all() is used only for user space mappings
and an ISB is not required because of an exception return before executing
user instructions. An exception return would behave like an ISB.
Signed-off-by: Vinayak Kale <vkale@apm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/cacheflush.h

@@ -116,6 +116,7 @@ extern void flush_dcache_page(struct page *);
 static inline void __flush_icache_all(void)
 {
 	asm("ic	ialluis");
+	dsb();
 }
 
 #define flush_dcache_mmap_lock(mapping) \