md/raid10: initialize r10_bio->read_slot before use.

In __make_request() a new r10bio is allocated and passed to
raid10_read_request(). The read_slot member of the bio is not
initialized, and the raid10_read_request() uses it to index an
array. This leads to occasional panics.

Fix by initializing the field to invalid value and checking for
valid value in raid10_read_request().

Cc: stable@vger.kernel.org
Signed-off-by: Kevin Vigor <kvigor@gmail.com>
Signed-off-by: Song Liu <songliubraving@fb.com>

drivers/md/raid10.c

@@ -1127,7 +1127,7 @@ static void raid10_read_request(struct mddev *mddev, struct bio *bio,
 	struct md_rdev *err_rdev = NULL;
 	gfp_t gfp = GFP_NOIO;
 
-	if (r10_bio->devs[slot].rdev) {
+	if (slot >= 0 && r10_bio->devs[slot].rdev) {
 		/*
 		 * This is an error retry, but we cannot
 		 * safely dereference the rdev in the r10_bio,
@@ -1508,6 +1508,7 @@ static void __make_request(struct mddev *mddev, struct bio *bio, int sectors)
 	r10_bio->mddev = mddev;
 	r10_bio->sector = bio->bi_iter.bi_sector;
 	r10_bio->state = 0;
+	r10_bio->read_slot = -1;
 	memset(r10_bio->devs, 0, sizeof(r10_bio->devs[0]) * conf->geo.raid_disks);
 
 	if (bio_data_dir(bio) == READ)