change os_trim() prototype not to use int fd

change os_trim() prototype not to use int fd

This is the same as the previous commit. The offset and length are
universal regardless of platforms, but int fd is not, thus it should
take struct fio_file*.

In fact, other fd related functions under os/ don't take int fd for
portability even if some of them work fine with int fd at the moment.

(OS headers basically (should)have no dependencies on fio functions
and structures, but struct fio_file is the only exception.)

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>