md raid10: fix NULL deference in handle_write_completed()

In the case of 'recover', an r10bio with R10BIO_WriteError &
R10BIO_IsRecover will be progressed by handle_write_completed().
This function traverses all r10bio->devs[copies].
If devs[m].repl_bio != NULL, it thinks conf->mirrors[dev].replacement
is also not NULL. However, this is not always true.

When there is an rdev of raid10 has replacement, then each r10bio
->devs[m].repl_bio != NULL in conf->r10buf_pool. However, in 'recover',
even if corresponded replacement is NULL, it doesn't clear r10bio
->devs[m].repl_bio, resulting in replacement NULL deference.

This bug was introduced when replacement support for raid10 was
added in Linux 3.3.

As NeilBrown suggested:
	Elsewhere the determination of "is this device part of the
	resync/recovery" is made by resting bio->bi_end_io.
	If this is end_sync_write, then we tried to write here.
	If it is NULL, then we didn't try to write.

Fixes: 9ad1aefc8a ("md/raid10:  Handle replacement devices during resync.")
Cc: stable (V3.3+)
Suggested-by: NeilBrown <neilb@suse.com>
Signed-off-by: Yufen Yu <yuyufen@huawei.com>
Signed-off-by: Shaohua Li <sh.li@alibaba-inc.com>

drivers/md/raid10.c

@@ -2655,7 +2655,8 @@ static void handle_write_completed(struct r10conf *conf, struct r10bio *r10_bio)
 		for (m = 0; m < conf->copies; m++) {
 			int dev = r10_bio->devs[m].devnum;
 			rdev = conf->mirrors[dev].rdev;
-			if (r10_bio->devs[m].bio == NULL)
+			if (r10_bio->devs[m].bio == NULL ||
+				r10_bio->devs[m].bio->bi_end_io == NULL)
 				continue;
 			if (!r10_bio->devs[m].bio->bi_status) {
 				rdev_clear_badblocks(
@@ -2670,7 +2671,8 @@ static void handle_write_completed(struct r10conf *conf, struct r10bio *r10_bio)
 					md_error(conf->mddev, rdev);
 			}
 			rdev = conf->mirrors[dev].replacement;
-			if (r10_bio->devs[m].repl_bio == NULL)
+			if (r10_bio->devs[m].repl_bio == NULL ||
+				r10_bio->devs[m].repl_bio->bi_end_io == NULL)
 				continue;
 
 			if (!r10_bio->devs[m].repl_bio->bi_status) {