mkfs.xfs: tweak wording of external log device size complaint

mkfs.xfs: tweak wording of external log device size complaint

If the external log device is too small to satisfy minimum requirements,
mkfs will complain about "external log device 512 too small...".  That
doesn't make any sense, so add a few missing words to clarify what we're
talking about:

"external log device size 512 blocks too small..."

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Allison Collins <allison.henderson@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>