xfs: create a static name for the dot entry too

Create an xfs_name_dot object so that upcoming scrub code can compare
against that.  Offline repair already has such an object, so we're
really just hoisting it to the kernel.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>