configfs: use size_t length modifier in pr_debug format argument

Signed-off-by: Zach Brown <zach.brown@oracle.com>
Acked-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>