Btrfs: fix missing skinny metadata check in scrub_stripe()

Check if we support skinny metadata firstly and fix to use
right type to search.

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>