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: NWill Deacon <will.deacon@arm.com>

arch/arm/include/asm/cacheflush.h

@@ -352,7 +352,7 @@ static inline void flush_cache_vmap(unsigned long start, unsigned long end)
 		 * 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)