[JFFS2] Fix printk format in jffs2_sum_write_data() error message.

fs/jffs2/summary.c: In function ‘jffs2_sum_write_data’:
fs/jffs2/summary.c:658: warning: format ‘%zd’ expects type ‘signed size_t’, but argument 4 has type ‘uint32_t’

Signed-off-by: David Woodhouse <dwmw2@infradead.org>