fio: move dynamic library handle to io_ops structure

fio: move dynamic library handle to io_ops structure

Keeping a dynamic engine's dlopen'd dlhandle on a thread structure doesn't
make sense; that thread may exit while others are still using the engine.

Move the dlhandle onto the ops structure itself.

We still only call dlopen for the first thead, which leaves a refcounting
issue which will be fixed in the next patch.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>