xfs_repair: add prefetch trace calls to debug thread creation failures

xfs_repair: add prefetch trace calls to debug thread creation failures

When debugging prefetch failures, it's useful to have thread creation
failure messages that are output as warnings on stderr in the trace
log as well.  It's also helpful to see when an AG gets queued behind
another one rather than having the thread started directly, which
has a separate trace line.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>