xfstests: generic/315: allow a little tolerance for our used check

xfstests: generic/315: allow a little tolerance for our used check

So df in btrfs is tricky at best, and relying on it for accurate information is
not great, but it's the best way to verify this test.  To get around btrfs being
inconsistent sometimes just use _within_tolerance to check our new df value to
make sure that our truncate did something.  With this patch I no longer see
transient failures of this test.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Reviewed-by: Jie Liu <jeff.liu@oracle.com>
Signed-off-by: Ben Myers <bpm@sgi.com>