loff_t is Linux-specific and unneeded at this place, as sizes are usually
denoted with a size_t. As the signedness of max_size isn't relied on
anywhere, replace it with a size_t.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>