md: fix possible oops when removing a bitmap from an active array

It is possible to add a write-intent bitmap to an active array, or remove
the bitmap that is there.

When we do with the 'quiesce' the array, which causes make_request to
block in "wait_barrier()".

However we are sampling the value of "mddev->bitmap" before the
wait_barrier call, and using it afterwards.  This can result in using a
bitmap structure that has been freed.
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/md/raid1.c

@@ -773,7 +773,7 @@ static int make_request(struct request_queue *q, struct bio * bio)
 	r1bio_t *r1_bio;
 	struct bio *read_bio;
 	int i, targets = 0, disks;
-	struct bitmap *bitmap = mddev->bitmap;
+	struct bitmap *bitmap;
 	unsigned long flags;
 	struct bio_list bl;
 	struct page **behind_pages = NULL;
@@ -802,6 +802,8 @@ static int make_request(struct request_queue *q, struct bio * bio)
 
 	wait_barrier(conf);
 
+	bitmap = mddev->bitmap;
+
 	disk_stat_inc(mddev->gendisk, ios[rw]);
 	disk_stat_add(mddev->gendisk, sectors[rw], bio_sectors(bio));