xfs: document log sysfs attributes in testing ABI

Create a sysfs-fs-xfs ABI documentation file for newly added sysfs
attributes. This is created under the testing section.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>