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

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")
Cc: stable@vger.kernel.org # 3.19+
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>