liburing.h: make all file/IO offset __u64
liburing.h: make all file/IO offset __u64

off_t may depend on various settings, it's safer to not truncate this
(temporarily) to an off_t as we use 64-bit types otherwise.

Fixes: https://github.com/axboe/liburing/issues/403
Signed-off-by: Jens Axboe <cd8c6775e60d6f67a6984377324e5290df3d5358@kernel.dk>

axboe/liburing,axboe/liburing,axboe/liburing