btrfs: add missing break in btrfs_print_leaf()

I don't think that BTRFS_DEV_EXTENT_KEY is supposed
to fall through to BTRFS_DEV_STATS_KEY ...

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>