md raid1: allow writebehind to work on any leg device set WriteMostly

As the WriteMostly flag can be set on any component device of a RAID1
array, remove the constraint that it only works if set on the first one.

Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com>
Tested-by: Xiao Ni <xni@redhat.com>
Link: https://lore.kernel.org/r/2a9592bf3340f34bf588eec984b23ee219f3985e.1692013451.git.heinzm@redhat.com
Signed-off-by: Song Liu <song@kernel.org>

Author: mauelsha

drivers/md/raid1.c

@@ -1523,8 +1523,7 @@ static void raid1_write_request(struct mddev *mddev, struct bio *bio,
 			 * Not if there are too many, or cannot
 			 * allocate memory, or a reader on WriteMostly
 			 * is waiting for behind writes to flush */
-			if (bitmap &&
-			    test_bit(WriteMostly, &rdev->flags) &&
+			if (bitmap && write_behind &&
 			    (atomic_read(&bitmap->behind_writes)
 			     < mddev->bitmap_info.max_write_behind) &&
 			    !waitqueue_active(&bitmap->behind_wait)) {