fat.c: fix printf() length modifier

The DIRENTSPERBLOCK utilizes sizeof() which will return a size_t which has no
fixed size. Therefor use correct length modifer for printf() statement to
prevent compiler warnings.

This patch fixes following warning:

---8<---
fat.c: In function 'do_fat_read':
fat.c:879: warning: format '%d' expects type 'int', but argument 4 has type 'long unsigned int'
--->8---

Signed-off-by: Andreas Bießmann <biessmann@corscience.de>
cc: Mike Frysinger <vapier@gentoo.org>
cc: Thomas Chou <thomas@wytron.com.tw>
cc: rjones@nexus-tech.net
cc: kharris@nexus-tech.net
Acked-by: Mike Frysinger <vapier@gentoo.org>