Use in-place path separator "/" for Linux specific code

Use in-place path separator "/" for Linux specific code

diskutil,cgroup,blktrace related code fully depend on Linux kernel,
so sprintf variants can use in-place path separator "/" instead of
FIO_OS_PATH_SEPARATOR.

In fact these Linux specific files are mix of two types within the
same file depending on who wrote them, where they were originally
using "/".

Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>