ARM: cacheflush: use -ishst dsb variant for ensuring flush completion

flush_cache_vmap contains a dsb to ensure that any cacheflushing
operations to flush out newly written ptes have completed.

This patch adds the -ishst option to the dsb, since that is all that is
required for completing cacheflushing in the inner-shareable domain.

Signed-off-by: Will Deacon <will.deacon@arm.com>
diff --git a/arch/arm/include/asm/cacheflush.h b/arch/arm/include/asm/cacheflush.h
index 17d0ae8..04d7326 100644
--- a/arch/arm/include/asm/cacheflush.h
+++ b/arch/arm/include/asm/cacheflush.h
@@ -352,7 +352,7 @@
 		 * set_pte_at() called from vmap_pte_range() does not
 		 * have a DSB after cleaning the cache line.
 		 */
-		dsb();
+		dsb(ishst);
 }
 
 static inline void flush_cache_vunmap(unsigned long start, unsigned long end)