null_blk: Fix missing mutex_destroy() at module removal

When a mutex lock is not used any more, the function mutex_destroy
should be called to mark the mutex lock uninitialized.

Fixes: f2298c04 ("null_blk: multi queue aware block test driver")
Signed-off-by: Zhu Yanjun <yanjun.zhu@linux.dev>
Link: https://lore.kernel.org/r/20240425171635.4227-1-yanjun.zhu@linux.dev


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