xfs: cleanup the definition of struct xfs_dir2_data_entry

Remove the tag member which is at a variable offset after the actual
name, and make name a real variable sized C99 array instead of the incorrect
one-sized array which confuses (not only) gcc.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>