erofs-utils: Convert from off64_t to off_t

Ensure largefile macros are used to determine size of off_t and not the
explict 64bit version of functions and types

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>