core:tftp: Use right correctly sized pointers

struct inode's size was changed to a uint64_t then preset to -1.  Setting
as a uint32_t only sets the low bits, resulting in an impossibly large
size and is rejected, leading to the file close return on TFTP.

Author:        Celelibi  <celelibi@gmail.com>
Signed-off-by: Gene Cumm <gene.cumm@gmail.com>