[XFS] fix spurious gcc warnings

Some recent gcc warnings don't like passing string variables to
printf-like functions without using at least a "%s" format string.
Change the two occurances of that in xfs to please gcc.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Eric Sandeen <sandeen@sandeen.net>
Signed-off-by: Niv Sardi <xaiki@sgi.com>