btrfs: Add btrfs: prefix to kernel log output

The kernel log entries for device label %s and device fsid %pU
are missing the btrfs: prefix. Add those here.

Signed-off-by: Frank Holton <fholton@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>