xfs: small cleanup in xfs_lowbit64()

There are two checkpatch.pl complaints here because of the bad
indenting and because of the assignment inside the condition.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>