dm thin metadata: don't issue prefetches if a transaction abort has failed

commit 2eae9e44 upstream.

If a transaction abort has failed then we can no longer use the metadata
device.  Typically this happens if the superblock is unreadable.

This fix addresses a crash seen during metadata device failure testing.

Fixes: 8a01a6af ("dm thin: prefetch missing metadata pages")
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

drivers/md/dm-thin-metadata.c

@@ -1935,5 +1935,8 @@ bool dm_pool_metadata_needs_check(struct dm_pool_metadata *pmd)
 
 void dm_pool_issue_prefetches(struct dm_pool_metadata *pmd)
 {
+	down_read(&pmd->root_lock);
+	if (!pmd->fail_io)
 		dm_tm_issue_prefetches(pmd->tm);
+	up_read(&pmd->root_lock);
 }