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: 8a01a6af75 ("dm thin: prefetch missing metadata pages")
Cc: stable@vger.kernel.org # 3.19+
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
2eae9e4489