Use off64_t instead of __off64_t

__off64_t is a kernel internal symbol, which happens to be user-visible
with glibc, but not necessarily with other C libraries such as musl.

Switching from __off64_t to off64_t allows the code to build properly
with musl.

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>