pstore/ram: Fix printk format warning

Fix printk format warning (on i386) in pstore:

fs/pstore/ram.c:409:3: warning: format '%lu' expects type 'long unsigned int', but argument 2 has type 'size_t'

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>