stat: used shared sem for stats lock

stat: used shared sem for stats lock

Like commit:

21628ec537c7 ("fio_sem, diskutil: introduce fio_shared_sem and use it for diskutil lock")

the stats sem is also potentially shared between processes, and hence
should be allocated and freed as a shared sem.

See the referenced commit, which has more details. Switch the stats sem
to be allocated in such a way that it's propagated properly between
processes.

Signed-off-by: Jens Axboe <axboe@kernel.dk>