raid1: copy write hint from master bio to behind bio

Signed-off-by: NMariusz Dabrowski <mariusz.dabrowski@intel.com>
Reviewed-by: NArtur Paszkiewicz <artur.paszkiewicz@intel.com>
Reviewed-by: NPawel Baldysiak <pawel.baldysiak@intel.com>
Signed-off-by: NShaohua Li <shli@fb.com>

drivers/md/raid1.c

@@ -1102,6 +1102,8 @@ static void alloc_behind_master_bio(struct r1bio *r1_bio,
 		goto skip_copy;
 	}
 
+	behind_bio->bi_write_hint = bio->bi_write_hint;
+
 	while (i < vcnt && size) {
 		struct page *page;
 		int len = min_t(int, PAGE_SIZE, size);