The prefix suggests the number should be printed in hex, so use
the %x specifier to do that.

Found by using regex suggested by Joe Perches.
Signed-off-by: NHans Wennborg <hans@hanshq.net>
Reviewed-by: NHannes Reinecke <hare@suse.de>
Signed-off-by: NChristoph Hellwig <hch@lst.de>